Airframe protection systems for use on rotorcraft

ABSTRACT

A yaw control system for a helicopter having an airframe that includes a tailboom includes one or more tail rotors rotatably coupled to the tailboom and a flight control computer implementing an airframe protection module. The airframe protection module includes an airframe protection monitoring module configured to monitor one or more flight parameters of the helicopter and an airframe protection command module configured to modify one or more operating parameters of the one or more tail rotors based on the one or more flight parameters of the helicopter, thereby protecting the airframe of the helicopter.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to yaw control systems foruse on aircraft and, in particular, to airframe protection systems toprotect the airframe of a rotorcraft from being damaged by the tailrotor(s) of a yaw control system.

BACKGROUND

The main rotor of a helicopter, which produces lift necessary forflight, also produces a counteracting torque force on the fuselage ofthe helicopter, turning the tailboom of the helicopter in the oppositedirection of the main rotor. The helicopter's tail rotor, located on atailboom aft of the main rotor, is used to counteract this torque andcontrol the yaw of the helicopter during flight. Tail rotors arenormally mounted on a horizontal axis perpendicular to the direction offlight of the helicopter. The tail rotor blades of conventional tailrotors change pitch to control anti-torque thrust direction andintensity, although some tail rotors include fixed pitch tail rotorblades. Most conventional tail rotors are rotationally driven by anengine mechanically linked to the tail rotor by a gearbox and adriveshaft extending through the tailboom.

Traditional tail rotors suffer from several drawbacks. For example,current tail rotors may be prone to blade stall within their operationalenvelopes. Also, the main rotor of a helicopter produces a transverseairflow while the tail rotor may be driven at high angular velocities toprovide adequate aerodynamic responses to such airflow. Sometimes,vortices produced by the main rotor of the helicopter and the tail rotorcan interact to reduce the efficiency of the thrust created by therotors. Traditional tail rotors can also be a significant source ofnoise in current helicopters, thus diminishing effectiveness when areduced noise environment is preferable such as during airreconnaissance or clandestine operations or in urban environments. Forexample, the interference of the vortices produced by the main rotor ofthe helicopter and the tail rotor may cause an increase in noise. Inaddition, although the anti-torque requirement of a helicopter issignificantly less in forward flight than in hover, many current tailrotors continue to rotate at high speed in forward flight, thusproducing unnecessary noise both directly by the spinning blades andindirectly via transmission noise. Some tail rotors may also pose athreat to the structural integrity of the helicopter. For example, sharpchanges in the anti-torque load of a tail rotor in some flightconditions may result in structural damage to the airframe of thehelicopter or to the tail rotor itself. The power consumptioncharacteristics of tail rotors can also vary dramatically depending onvarious factors such as the flight mode of the helicopter. Currenthelicopters require power sources that are sized to provide peak powervalues even though such power values are unnecessary in most modes offlight. In addition, for helicopters having multiple tail rotors, thepower consumption, load or other operating parameters of the respectivetail rotors can differ from one another, sometimes causing severeimbalances affecting the efficiency of the tail rotors. Currenthelicopters fail to balance such operating parameters of the tail rotorsduring flight to improve efficiency. Accordingly, a need has arisen forimproved yaw control systems that address these and other drawbacks ofcurrent tail rotors.

SUMMARY

In a first aspect, the present disclosure is directed to a yaw controlsystem for a helicopter having an airframe that includes a tailboom. Theyaw control system includes one or more tail rotors rotatably coupled tothe tailboom and a flight control computer implementing an airframeprotection module. The airframe protection module includes an airframeprotection monitoring module configured to monitor one or more flightparameters of the helicopter and an airframe protection command moduleconfigured to modify one or more operating parameters of the one or moretail rotors based on the one or more flight parameters of thehelicopter, thereby protecting the airframe of the helicopter.

In some embodiments, the airframe protection monitoring module mayinclude an airspeed monitoring module configured to monitor the airspeedof the helicopter, the airframe protection command module configured tomodify the one or more operating parameters of the one or more tailrotors based on the airspeed of the helicopter. In certain embodiments,the airframe protection command module may be configured to modify theone or more operating parameters of the one or more tail rotors inresponse to the airspeed of the helicopter exceeding an airspeedthreshold. In some embodiments, the airframe protection monitoringmodule may include a maneuver detection module configured to detect amaneuver being performed by the helicopter, the airframe protectioncommand module configured to modify the one or more operating parametersof the one or more tail rotors based on the maneuver. In certainembodiments, the maneuver detection module may be configured to detect asideward flight maneuver, the airframe protection command moduleconfigured to modify the one or more operating parameters of the one ormore tail rotors in response to the maneuver detection module detectingthe sideward flight maneuver. In some embodiments, the airframeprotection monitoring module may include a load determination moduleconfigured to detect a load on the airframe of the helicopter, theairframe protection command module configured to modify the one or moreoperating parameters of the one or more tail rotors based on the load onthe airframe. In certain embodiments, the one or more tail rotors eachinclude tail rotor blades and the airframe protection monitoring modulemay include a load determination module configured to detect a load onthe tail rotor blades, the airframe protection command module configuredto modify the one or more operating parameters of the one or more tailrotors based on the load on the tail rotor blades.

In some embodiments, the one or more tail rotors each include tail rotorblades and the airframe protection monitoring module may include a tailrotor blade clearance monitoring module configured to detect a clearancedistance between the tail rotor blades and the airframe of thehelicopter, the airframe protection command module configured to modifythe one or more operating parameters of the one or more tail rotorsbased on the clearance distance. In certain embodiments, the airframeprotection command module may be configured to modify the one or moreoperating parameters of the one or more tail rotors in response to theclearance distance being less than a minimum tail rotor blade clearancethreshold. In some embodiments, the airframe protection command modulemay include an acceleration regulator to modify a maximum angularacceleration of the one or more tail rotors, the acceleration regulatorlimiting angular acceleration of the one or more tail rotors to themaximum angular acceleration based on the one or more flight parametersof the helicopter. In certain embodiments, the airframe protectioncommand module may include an acceleration regulator configured tomodify a maximum angular deceleration of the one or more tail rotors,the acceleration regulator limiting angular deceleration of the one ormore tail rotors to the maximum angular deceleration based on the one ormore flight parameters of the helicopter. In some embodiments, theairframe protection command module may include a rotational speedregulator configured to modify a maximum rotational speed of the one ormore tail rotors, the rotational speed regulator limiting a rotationalspeed of the one or more tail rotors to the maximum rotational speedbased on the one or more flight parameters of the helicopter. In certainembodiments, the airframe protection command module may include arotational speed regulator configured to reduce the rotational speed ofthe one or more tail rotors based on the one or more flight parametersof the helicopter.

In a second aspect, the present disclosure is directed to a rotorcraftincluding a fuselage, a tailboom extending from the fuselage and a yawcontrol system. The yaw control system includes a shroud coupled to theaft portion of the tailboom and forming one or more ducts, one or moretail rotors disposed in the one or more ducts and a flight controlcomputer implementing an airframe protection module. The airframeprotection module includes an airframe protection monitoring moduleconfigured to monitor one or more flight parameters of the rotorcraftand an airframe protection command module configured to modify one ormore operating parameters of the one or more tail rotors based on theone or more flight parameters of the rotorcraft, thereby protecting anairframe of the rotorcraft.

In some embodiments, each tail rotor may include tail rotor blades and amotor secured by one or more stators within a respective one of theducts. In such embodiments, the airframe protection command module maybe configured to modify the one or more operating parameters of the oneor more tail rotors based on the one or more flight parameters of therotorcraft to prevent contact between the tail rotor blades and the oneor more stators. In certain embodiments, the one or more tail rotors mayeach include variable pitch tail rotor blades and the airframeprotection command module may include a blade pitch regulator configuredto modify a blade pitch of the variable pitch tail rotor blades based onthe one or more flight parameters of the rotorcraft.

In a third aspect, the present disclosure is directed a method forprotecting an airframe of a helicopter including monitoring one or moreflight parameters of the helicopter; and modifying one or more operatingparameters of one or more tail rotors of the helicopter based on the oneor more flight parameters, thereby protecting the airframe of thehelicopter.

In some embodiments, monitoring the one or more flight parameters of thehelicopter may include monitoring the airspeed of the helicopter. Insuch embodiments, modifying the one or more operating parameters of theone or more tail rotors of the helicopter based on the one or moreflight parameters may include modifying the one or more operatingparameters of the one or more tail rotors of the helicopter in responseto the airspeed of the helicopter exceeding an airspeed threshold. Incertain embodiments, monitoring the one or more flight parameters of thehelicopter may include monitoring the airspeed of the helicopter, themaneuver being performed by the helicopter, the load experienced by thehelicopter and/or the clearance distance between the airframe of thehelicopter and tail rotor blades of the one or more tail rotors. In suchembodiments, modifying the one or more operating parameters of the oneor more tail rotors of the helicopter may include modifying the maximumangular acceleration, the maximum angular deceleration, the maximumrotational speed, the rotational speed and/or the blade pitch of the oneor more tail rotors. In some embodiments, modifying the one or moreoperating parameters of the one or more tail rotors of the helicoptermay include modifying anti-torque input from an operator of thehelicopter based on the one or more flight parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1C are schematic illustrations of a rotorcraft having a yawcontrol system in accordance with embodiments of the present disclosure;

FIG. 2 is a side view of an anti-torque system used on previousaircraft;

FIGS. 3A-3B are block diagrams of a propulsion and control system for arotorcraft having a yaw control system in accordance with embodiments ofthe present disclosure;

FIG. 4 is a block diagram of a control system for a rotorcraft having ayaw control system in accordance with embodiments of the presentdisclosure;

FIGS. 5A-5B are various views of a rotorcraft having a yaw controlsystem including a yaw control matrix having various configurations inaccordance with embodiments of the present disclosure;

FIGS. 6A-6M are various views of yaw control systems for rotorcraftincluding yaw control matrices having different configurations inaccordance with embodiments of the present disclosure;

FIGS. 7A-7D are schematic illustrations of a yaw control system for arotorcraft including a rudder in accordance with embodiments of thepresent disclosure;

FIGS. 8A-8C are various views of a yaw control system for a rotorcraftincluding multiple rudders in accordance with embodiments of the presentdisclosure;

FIGS. 9A-9B are top views of a rotorcraft having a yaw control systemincluding a rotatable yaw control matrix in accordance with embodimentsof the present disclosure;

FIGS. 10A-10B are side views of yaw control systems for rotorcraftincluding yaw control matrices and rudders having differentconfigurations in accordance with embodiments of the present disclosure;

FIGS. 11A-11B are flowcharts of various methods for controlling the yawof a rotorcraft using a rudder in accordance with embodiments of thepresent disclosure;

FIGS. 12A-12B are schematic illustrations of a yaw control system for arotorcraft having a quiet mode in accordance with embodiments of thepresent disclosure;

FIGS. 13A-13D are various views of a rotorcraft having a yaw controlsystem in different flight operating scenarios in accordance withembodiments of the present disclosure;

FIGS. 14A-14B are isometric views of an electric vertical takeoff andlanding aircraft having a quiet mode in accordance with embodiments ofthe present disclosure;

FIG. 15 is a flowchart of a method for managing noise emissions from oneor more tail rotors of a rotorcraft in accordance with embodiments ofthe present disclosure;

FIGS. 16A-16B are schematic illustrations of a yaw control system for arotorcraft including an airframe protection system in accordance withembodiments of the present disclosure;

FIGS. 17A-17C are top views of a rotorcraft having a yaw control systemincluding an airframe protection system in various flight operatingscenarios in accordance with embodiments of the present disclosure;

FIGS. 18A-18B are flowcharts of various methods for protecting theairframe of a rotorcraft having a yaw control system in accordance withembodiments of the present disclosure;

FIG. 19 is a schematic illustration of a yaw control system for arotorcraft including a power management system in accordance withembodiments of the present disclosure;

FIGS. 20A-20H are schematic illustrations of a rotorcraft having a yawcontrol system including a power management system in various flightoperating scenarios in accordance with embodiments of the presentdisclosure;

FIG. 21 is a schematic illustration of a power management system for arotorcraft having a yaw control system in accordance with embodiments ofthe present disclosure;

FIGS. 22A-22B are flowcharts of various methods for managing power foran electrically distributed yaw control system of a rotorcraft inaccordance with embodiments of the present disclosure;

FIG. 23 is a schematic illustration of a yaw control system for arotorcraft including a tail rotor balancing system in accordance withembodiments of the present disclosure;

FIGS. 24A-24D are various views of a rotorcraft having a yaw controlsystem including a tail rotor balancing system in various flightoperating scenarios in accordance with embodiments of the presentdisclosure;

FIGS. 25A-25B are isometric views of an electric vertical takeoff andlanding aircraft having a tail rotor balancing system in accordance withembodiments of the present disclosure; and

FIGS. 26A-26C are flowcharts of various methods for operating the tailrotors of a rotorcraft having a yaw control system in accordance withembodiments of the present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,all features of an actual implementation may not be described in thisspecification. It will of course be appreciated that in the developmentof any such actual embodiment, numerous implementation-specificdecisions must be made to achieve the developer's specific goals, suchas compliance with system-related and business-related constraints,which will vary from one implementation to another. Moreover, it will beappreciated that such a development effort might be complex andtime-consuming but would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicesdescribed herein may be oriented in any desired direction. As usedherein, the term “coupled” may include direct or indirect coupling byany means, including by mere contact or by moving and/or non-movingmechanical connections.

Referring to FIGS. 1A-1C in the drawings, a rotorcraft is schematicallyillustrated and generally designated 10. In the illustrated embodiment,rotorcraft 10 is depicted as a helicopter having a main rotor 12, whichincludes a plurality of rotor blades 14. Main rotor 12 is rotatablerelative to a fuselage 16. The pitch of rotor blades 14 can becollectively and/or cyclically manipulated to selectively controldirection, thrust and lift of rotorcraft 10. A landing gear system 18provides ground support for rotorcraft 10. A tailboom 20 extends in anaft direction from fuselage 16. Main rotor 12 rotates in a directionindicated by arrow 22 a, which produces a torque on fuselage 16 in adirection indicated by arrow 22 b. Rotorcraft 10 has a yaw controlsystem 24 including a yaw control matrix 26 coupled to an aft portion oftailboom 20. Yaw control system 24 produces anti-torque thrust 22 c tocounteract torque 22 b and generally controls the yaw of rotorcraft 10in various flight modes.

Referring to FIG. 2 in conjunction with FIGS. 1A-1C in the drawings, ananti-torque system 28 used in previous aircraft is depicted. Traditionalanti-torque systems such as anti-torque system 28 include a single tailrotor 30 having variable pitch tail rotor blades 30 a. Tail rotor 30 isrotated at a fixed rotational speed by a mechanical drivetrain 32including an engine 32 a, a gearbox 32 b and one or more driveshafts 32c. Tail rotor 30 also includes a pitch control assembly (not shown) thatadjusts the pitch of variable pitch tail rotor blades 30 a to vary theanti-torque thrust produced by tail rotor 30, thereby controlling theyaw of the helicopter during flight. Anti-torque system 28 suffers fromseveral drawbacks. For example, tail rotor 30 may be prone to bladestall within its operational envelope. In addition, main rotor 34produces a transverse airflow. Tail rotor 30 must sometimes be driven athigh angular velocities to provide adequate aerodynamic responses tosuch airflow. Vortices produced by tail rotor 30 and main rotor 34 caninteract to reduce the efficiency of the thrust produced by both rotors30, 34. Tail rotor 30 can also be a significant source of noise, thusdiminishing effectiveness when a reduced noise environment is preferablesuch as during air reconnaissance or clandestine operations. Forexample, the interference of the vortices produced by tail rotor 30 andmain rotor 34 may cause an increase in noise. In addition, tail rotor 30continues to rotate at high speed in forward flight, thus producingunnecessary noise generated both directly by the spinning of tail rotorblades 30 a and indirectly via noise from gearbox 32 b. Tail rotor 30may also pose a threat to the structural integrity of the helicopter.For example, sharp changes in the anti-torque load of tail rotor 30 insome flight conditions may result in structural damage to the airframeof the helicopter or to tail rotor 30 itself. The power consumptioncharacteristics of tail rotor 30 can also vary dramatically depending onvarious factors such as the flight mode of the helicopter. Currenthelicopters require power sources that are sized to provide peak powervalues even though such power values are unnecessary in most modes offlight.

Referring back to FIGS. 1A-1C, yaw control system 24 of rotorcraft 10addresses these and other drawbacks of current anti-torque systems. Yawcontrol matrix 26 of yaw control system 24 includes a shroud 36 having agenerally parallelogram or rhombus shape configuration, although shroud36 may form any general shape. Shroud 36 is vertically oriented andtherefore substantially aligned with centerline 38 of rotorcraft 10 whenviewed from the front. In other embodiments, shroud 36 may be canted, orform an angle, relative to centerline 38. Shroud 36 forms ducts 40 inwhich tail rotors 42, 44, 46, 48 are disposed including upper forwardtail rotor 42, upper aft tail rotor 44, lower forward tail rotor 46 andlower aft tail rotor 48. While four tail rotors 42, 44, 46, 48 areillustrated, yaw control system 24 may include any number of tail rotorssuch as two, three, five or more tail rotors. Shroud 36 increases safetyfor ground personnel and crew by preventing contact with spinning tailrotors 42, 44, 46, 48. It will be appreciated by one of ordinary skillin the art, however, that all or some of tail rotors 42, 44, 46, 48 maybe open or non-ducted tail rotors that lack a shroud.

As illustrated, yaw control system 24 is an electrically distributed yawcontrol system in which each tail rotor 42, 44, 46, 48 has fixed pitchtail rotor blades 42 a, 44 a, 46 a, 48 a and a variable rotational speedmotor 42 b, 44 b, 46 b, 48 b secured by stators, or spokes, 50. Eachmotor 42 b, 44 b, 46 b, 48 b may be operated individually or in anycombination in either or both rotational directions to provideanti-torque or pro-torque forces for stabilizing the yaw of rotorcraft10. In other embodiments, tail rotors 42, 44, 46, 48 may be variablepitch, fixed rotational speed or variable pitch, variable rotationalspeed tail rotor systems that are electrically, mechanically orhydraulically driven. In such embodiments, each tail rotor 42, 44, 46,48 may include a pitch control assembly driven by an electrical orhydraulic actuator that changes the pitch of tail rotor blades 42 a, 44a, 46 a, 48 a to adjust thrust. In other embodiments, the pitch controlassembly may also be operated with mechanical linkages, bell cranksand/or other mechanical members without any actuators. It will beappreciated by one of ordinary skill in the art in view of theillustrative embodiments disclosed herein that yaw control matrix 26 mayhave any number of ducts and tail rotors forming, along with shroud 36,any shape or spatial positioning configuration. Yaw control system 24may also include a combination of fixed pitch and variable pitch tailrotors. Yaw control system 24 includes a vertical fin 52, of which a topportion 52 a is coupled to the top side of shroud 36 and a smallerbottom portion 52 b is coupled to the bottom side of shroud 36. Bottomportion 52 b of vertical fin 52 may be utilized as a bumper duringtakeoff and landing. A horizontal stabilizer 54 coupled to tailboom 20includes stabilizer fins 54 a to provide additional yaw stability inforward flight.

Yaw control system 24 manages the yaw of rotorcraft 10 throughout thevarious flight modes of rotorcraft 10. For example, in hover mode yawcontrol matrix 26 manages the yaw of rotorcraft 10 by producinganti-torque thrust 22 c to counteract torque 22 b on fuselage 16 causedby rotation 22 a of main rotor 12. In hover mode yaw control matrix 26may also provide maneuverability and trim to the operations ofrotorcraft 10. Yaw control matrix 26 may also produce pro-torque thrustin the same direction as torque 22 b on fuselage 16 for certainmaneuvers such as quick turn maneuvers. During hover, the thrust vectorsproduced by tail rotors 42, 44, 46, 48 may be in a uniform or nonuniformdirection. For example, one set of one or more tail rotors 42, 44, 46,48 may direct their thrust in one direction such as in anti-torquedirection 22 c while a different set of one or more tail rotors 42, 44,46, 48 direct their thrust in the opposite direction such as apro-torque direction to provide finer rotational control for rotorcraft10. The speed of each motor 42 b, 44 b, 46 b, 48 b may also be variedunder the control of logic in a flight control computer 56 thatcalculates the optimum thrust magnitude for each tail rotor 42, 44, 46,48 to achieve a desired yaw orientation. Flight control computer 56 alsoimplements one or more systems or modules that control the operation ofyaw control system 24 to achieve various objectives such as noisereduction, thrust efficiency, airframe protection, load balancing andpower consumption balancing, among others. Vertical fin 52 andstabilizer fins 54 a provide rotorcraft 10 with yaw stability in forwardflight. Yaw control matrix 26 may be selectively engaged to emit variousthrust magnitudes in either direction during forward flight depending onthe yaw adjustment requirements of rotorcraft 10. Yaw control matrix 26also provides yaw stability for rotorcraft 10 during maneuvers such assideward flight, sharp turns, climbs and descents.

It should be appreciated that rotorcraft 10 is merely illustrative of avariety of aircraft that can implement the embodiments disclosed herein.Indeed, the illustrative embodiments of yaw control system 24 describedherein may be implemented on any aircraft that experiences yaw movement.Other aircraft implementations can include vertical takeoff and landing(VTOL) aircraft including electric VTOL (eVTOL) aircraft, hybridaircraft, tiltrotor aircraft, tiltwing aircraft, quad tiltrotoraircraft, unmanned aircraft, gyrocopters, drones, quadcopters, airplanesincluding propeller-driven airplanes, compound helicopters, jets and thelike. The illustrative embodiments of yaw control system 24 describedherein may also be utilized on any rotorcraft having a distributedpropulsion system with two or more rotors powered by an electrical,hydraulic, mechanical or other energy source. As such, those skilled inthe art will recognize that the illustrative embodiments of yaw controlsystem 24 described herein can be integrated into a variety of aircraftconfigurations. It should be appreciated that even though aircraft areparticularly well-suited to implement the embodiments of the presentdisclosure, non-aircraft vehicles and devices can also implement theembodiments.

Referring to FIGS. 3A-3B in the drawings, various systems of rotorcraft10 are depicted. Main rotor 12 includes a rotor hub 12 a with rotorblades 14 radiating therefrom and an electronics node 12 b including,for example, controllers 12 c, sensors 12 d and communications elements12 e, as well as other components suitable for use in the operation ofmain rotor 12. In some embodiments, rotor hub 12 a includes one or moreactuators (not shown) to adjust the collective and/or cyclic pitch ofrotor blades 14. Fuselage 16 houses a power system 58 including anengine 58 a, generator 58 b and one or more batteries 58 c to power thevarious systems of rotorcraft 10. Main rotor 12 is powered by engine 58a via a drivetrain that may include one or more gearboxes anddriveshafts.

Rotorcraft 10 also includes yaw control matrix 26 coupled to tailboom 20depicted as upper forward tail rotor 42, upper aft tail rotor 44, lowerforward tail rotor 46 and lower aft tail rotor 48. Each tail rotor 42,44, 46, 48 includes tail rotor blades 42 a, 44 a, 46 a, 48 a radiatingfrom a tail rotor hub 42 c, 44 c, 46 c, 48 c. Each tail rotor 42, 44,46, 48 also includes electronics nodes 42 d, 44 d, 46 d, 48 d depictedas having one or more controllers 42 e, 44 e, 46 e, 48 e such as a motorcontroller, one or more sensors 42 f, 44 f, 46 f, 48 f that may be used,for example, to detect operating parameters of tail rotors 42, 44, 46,48 and communications elements 42 g, 44 g, 46 g, 48 g enabling tailrotors 42, 44, 46, 48 to send and receive data amongst each other andwith other parts of rotorcraft 10. Each tail rotor 42, 44, 46, 48includes at least one variable speed electric motor 42 b, 44 b, 46 b, 48b. Tail rotor hubs 42 c, 44 c, 46 c, 48 c are coupled to the outputdrives of motors 42 b, 44 b, 46 b, 48 b. Tail rotor blades 42 a, 44 a,46 a, 48 a are fixed pitch tail rotor blades that emit variable thrustproportional to the rotational speeds of motors 42 b, 44 b, 46 b, 48 b.In other embodiments, tail rotors 42, 44, 46, 48 may include variablepitch tail rotor blades that are electrically, mechanically orhydraulically driven at a fixed or variable rotational speed with thevariable pitch tail rotor blades being actuated using an electrical ormechanical pitch control system. Power system 58 serves as theelectrical energy source for tail rotors 42, 44, 46, 48 including motors42 b, 44 b, 46 b, 48 b and electronics nodes 42 d, 44 d, 46 d, 48 d. Forexample, tail rotors 42, 44, 46, 48 may be powered by generator 58 band/or battery 58 c.

Battery 58 c may be charged by an electrical energy generation systemsuch as engine 58 a and generator 58 b or may be charged at a groundstation. Battery 58 c may also be interchangeably removed and installedto enable efficient refueling which may be particularly beneficial inembodiments of rotorcraft 10 wherein the sole electrical energy sourceis battery 58 c. In embodiments that include an electrical energygeneration system such as engine 58 a and generator 58 b housed withinfuselage 16, the electrical energy generation system may include one ormore fuel tanks such as liquid fuel tanks. In one non-limiting example,engine 58 a may be used to drive electric generator 58 b, which produceselectrical energy that is fed to tail rotors 42, 44, 46, 48 to powermotors 42 b, 44 b, 46 b, 48 b and electronics nodes 42 d, 44 d, 46 d, 48d. Rotorcraft 10 may implement a hybrid power system including bothengine 58 a and battery 58 c. This type of hybrid power system may bebeneficial in that the energy density of liquid fuel exceeds that ofbatteries enabling greater endurance for rotorcraft 10. In the hybridpower system, battery 58 c may provide a backup electrical power sourceto enable rotorcraft 10 to safely land in the event of a failure ofengine 58 a. In other embodiments, all propulsion systems of rotorcraft10 including main rotor 12 and tail rotors 42, 44, 46, 48 are poweredexclusively by battery 58 c. In yet other embodiments, each tail rotor42, 44, 46, 48 may include a respective battery to provide backupbattery power in the event of a failure of power system 58. As anotheralternative, tail rotors 42, 44, 46, 48 may include hydraulic motorsoperating within a common hydraulic fluid system wherein one or morehigh pressure hydraulic sources or generators are housed within fuselage16 to provide power to each of the hydraulic motors.

Rotorcraft 10 includes a flight control system 60 housed within fuselage16. Flight control system 60, such as a digital flight control system,may preferably be a redundant flight control system and more preferablya triply redundant flight control system having independent flightcontrol computers including flight control computer 56. Use of a dual,triple or more redundant flight control system 60 improves the overallsafety and reliability of rotorcraft 10 in the event of a failure offlight control system 60. Flight control system 60 preferably includesnon-transitory computer readable storage media including a set ofcomputer instructions executable by one or more processors forcontrolling the operation of main rotor 12 and yaw control system 24.Flight control system 60 may be implemented on one or more generalpurpose computers, special purpose computers or other machines withmemory or processing capability. For example, flight control system 60may include one or more memory storage modules including, but notlimited to, internal storage memory such as random access memory,non-volatile memory such as read only memory, removable memory such asmagnetic storage memory, optical storage, solid-state storage memory orother suitable memory storage. Flight control system 60 may be amicroprocessor-based system operable to execute program code in the formof machine executable instructions. In addition, flight control system60 may be selectively connectable to other computer systems via aproprietary encrypted network, a public encrypted network, the Internetor other suitable communication network that may include both wired andwireless connections.

Flight control system 60 may be in electrical, mechanical, wired,wireless, computer, hydraulic or any other type of communication withelectronics node 12 b of main rotor 12, electronics nodes 42 d, 44 d, 46d, 48 d of tail rotors 42, 44, 46, 48 and other parts of rotorcraft 10.For example, flight control system 60 may communicate via a wired and/orwireless communications network with electronics nodes 42 d, 44 d, 46 d,48 d of tail rotors 42, 44, 46, 48. In some embodiments, any combinationof electronics nodes 12 b, 42 d, 44 d, 46 d, 48 d may instead becombined and centralized into fuselage 16. Flight control system 60receives sensor data from and sends flight command information toelectronics nodes 42 d, 44 d, 46 d, 48 d of tail rotors 42, 44, 46, 48such that each tail rotor 42, 44, 46, 48 may be individually andindependently controlled and operated. Flight control system 60 isconfigured to receive inputs from flight sensors 12 d, 42 f, 44 f, 46 f,48 f such as, but not limited to, gyroscopes, accelerometers or anyother suitable sensing equipment configured to provide flight controlsystem 60 with spatial, positional or force dynamics information,feedback or other data that may be utilized to manage the operation ofrotorcraft 10. For example, flight control system 60 may use sensor datafrom flight sensors 42 f to generate and send flight command informationto electronics node 42 d to control upper forward tail rotor 42.

Rotorcraft 10 may include global positioning system 62 configured todetermine, receive and/or provide data related to the location ofrotorcraft 10 including flight destinations, targets, no-fly zones,preplanned routes, flight paths or any other geospatial location-relatedinformation. Global positioning system 62 may be configured forbidirectional communication with flight control system 60,unidirectional communication from global positioning system 62 to flightcontrol system 60 or unidirectional communication from flight controlsystem 60 to global positioning system 62. Rotorcraft 10 may includewireless communication components 64 such as radio communicationequipment configured to send and receive signals related to flightcommands or other operational information. Wireless communicationcomponents 64 may be configured to transmit video, audio or other datagathered, observed or otherwise generated, carried by or obtained byrotorcraft 10. In some embodiments, flight control system 60 may also beoperable to communicate with one or more remote systems via wirelesscommunication components 64 using a wireless communications protocol.The remote systems may be operable to receive flight data from andprovide commands to flight control system 60 to enable flight controlover some or all aspects of flight operation. In other embodiments,rotorcraft 10 may instead be a manned or piloted vehicle. In both mannedand unmanned missions, flight control system 60 may autonomously controlsome or all aspects of flight operation. Transitions between the variousflight modes may be accomplished responsive to remote flight control,autonomous flight control, onboard pilot flight control or combinationsthereof.

Flight control computer 56 implements one or more modules to control theoperation of yaw control matrix 26. For example, flight control computer56 includes a yaw controller 66 to determine the amount by which tochange or correct the yaw of rotorcraft 10 and selectively activate tailrotors 42, 44, 46, 48 at various thrust magnitudes to achieve thedesired yaw of rotorcraft 10. In some embodiments, yaw control system 24includes one or more vertical control surfaces such as a rudderrotatably mounted adjacent to yaw control matrix 26. In suchembodiments, yaw controller 66 may rotate the rudder in various flightmodes of rotorcraft 10 such as the forward flight mode to control theyaw of rotorcraft 10. Flight control computer 56 includes a quiet modecontroller 68 configured to selectively activate a quiet mode based oncertain flight parameters of rotorcraft 10 to reduce the noise emittedby yaw control matrix 26. Flight control computer 56 includes anairframe protection module 70 that adjusts certain operating parametersof tail rotors 42, 44, 46, 48 to protect the airframe of rotorcraft 10.Flight control computer 56 may also include a power management module 72configured to allocate power between the various elements of powersystem 58 and tail rotor motors 42 b, 44 b, 46 b, 48 b based on certainflight parameters of rotorcraft 10. Flight control computer 56 includesa tail rotor balancing module 74 to balance the power consumption oftail rotors 42, 44, 46, 48 and/or load experienced by tail rotors 42,44, 46, 48 based on certain flight parameters of rotorcraft 10 and/oroperating parameters of tail rotors 42, 44, 46, 48.

Referring additionally to FIG. 4 in the drawings, a block diagramdepicts a control system 76 operable for use with rotorcraft 10 of thepresent disclosure. In the illustrated embodiment, control system 76includes three primary computer based subsystems; namely, an airframesystem 78, a remote system 80 and a pilot system 82. In someimplementations, remote system 80 includes a programming application 80a and a remote control application 80 b. Programming application 80 aenables a user to provide a flight plan and mission information torotorcraft 10 such that flight control computer 56 may engage inautonomous control over rotorcraft 10. For example, programmingapplication 80 a may communicate with flight control computer 56 over awired or wireless communication channel 84 to provide a flight planincluding, for example, a starting point, a trail of waypoints and anending point such that flight control computer 56 may use waypointnavigation during the mission.

In the illustrated embodiment, flight control computer 56 is a computerbased system that includes one or more command modules 86 and one ormore monitoring modules 88. Flight control computer 56 may includeredundant command and/or monitoring modules. It is to be understood bythose skilled in the art that these and other modules executed by flightcontrol computer 56 may be implemented in a variety of forms includinghardware, software, firmware, special purpose processors andcombinations thereof. Flight control computer 56 receives input from avariety of sources including internal sources such as sensors 90,controllers and actuators 92, main rotor 12, tail rotors 42, 44, 46, 48and external sources such as remote system 80 as well as globalpositioning system satellites or other location positioning systems andthe like. Sensors 90 may include redundant sensors. During the variousflight modes of rotorcraft 10 including hover mode, forward flight modeand transitions therebetween, command module 86 provides commands tocontrollers and actuators 92 of main rotor 12 and tail rotors 42, 44,46, 48. These commands enable independent operation of each tail rotor42, 44, 46, 48 such as rotational speed, blade pitch and rotationaldirection as well as others depending on the tail rotor type. Flightcontrol computer 56 receives feedback and sensor measurements fromsensors 90, controllers and actuators 92, main rotor 12 and tail rotors42, 44, 46, 48. This feedback is processed by monitoring module 88,which can supply data and other information to command module 86 and/orcontrollers and actuators 92. Sensors 90, such as strain sensors,distance sensors, accelerometers, vibration sensors, location sensors,attitude sensors, altitude sensors, airspeed sensors, environmentalsensors, fuel sensors, temperature sensors and the like also provideinformation to flight control computer 56 to further enhance autonomouscontrol capabilities.

Monitoring module 88 includes a yaw monitoring module 66 a to determinean amount by which to change or correct the yaw of rotorcraft 10. Indetermining the yaw adjustment for rotorcraft 10, yaw monitoring module66 a may utilize sensors 90 such as a yaw rate sensor and/or a yawposition sensor. A yaw command module 66 b of command module 86 modifiesthe yaw of rotorcraft 10 using tail rotors 42, 44, 46, 48. Yaw commandmodule 66 b may determine the magnitudes of the thrusts from tail rotors42, 44, 46, 48 that are required to achieve the desired yaw ofrotorcraft 10 as determined by yaw monitoring module 66 a. Yaw commandmodule 66 b may also determine how quickly anti-torque or pro-torquethrust must be implemented so that the desired yaw is achieved in atimely manner. Yaw command module 66 b may also determine whether andhow fast to rotate each tail rotor 42, 44, 46, 48, taking into accountvarious characteristics of each tail rotor 42, 44, 46, 48 such as motorsize, diameter, position and blade pitch, as well as others. Yaw commandmodule 66 b thus enhances yaw management of rotorcraft 10 by selectivelyactivating tail rotors 42, 44, 46, 48 depending on the thrust,responsiveness, noise, power, load and/or structural requirements of theoperational circumstance. In one non-limiting example, yaw monitoringmodule 66 a receives measurements related to the yaw rotation ofrotorcraft 10 from sensors 90 such as a rotation sensor. Yaw commandmodule 66 b may change the respective speed of the variable speed motorsof tail rotors 42, 44, 46, 48 to adjust anti-torque or pro-torque thrustto achieve a desired rotation of rotorcraft 10, which may include norotation.

Yaw command module 66 b may reference a lookup table of known orestimated torque calculations or formulas for each of the variable speedmotors of tail rotors 42, 44, 46, 48 depending on the size of the motor,blade pitch, rotational direction, position in yaw control matrix 26 orother characteristics. The position of each tail rotor 42, 44, 46, 48 inyaw control matrix 26 affects their individual contribution to therotation of rotorcraft 10. For example, assuming all the variable speedmotors and fixed pitch blades are of equivalent size and power, then thevariable speed motors and fixed pitch blades in the aft-most positionwill have the greatest effect on rotorcraft anti-torque, while variablespeed motors and fixed pitch blades that are forward of other motorswill have less overall effect on rotorcraft anti-torque assumingequivalent rotational speed. As such, yaw command module 66 b may lookup the estimated or measured effect on anti-torque for each individualtail rotor motor and then increase or decrease the rotational speed ofany combination of tail rotors 42, 44, 46, 48 to adjust the yaworientation of rotorcraft 10. Yaw monitoring module 66 a may receivedata from a rotation sensor that reflects actual rotorcraft yaw rotationand yaw command module 66 b may compare the estimated or calculatedrotation of rotorcraft 10 versus actual rotation and then adjust therotational speed of one or more of the variable speed motors of tailrotors 42, 44, 46, 48 to control yaw orientation. In some embodiments,yaw command module 66 b may be in data communication with a table thatincludes the calculated rotorcraft anti-torque versus tail rotor speedfor each variable speed motor of tail rotors 42, 44, 46, 48. Yaw commandmodule 66 b may look up estimated anti-torques for the variable speedmotors to adjust the rotational speeds of the motors based on cockpitpedal input for overall rotorcraft rotation, then measure actualrotation and finally adjust the rotational speed of one or more of thevariable speed motors of tail rotors 42, 44, 46, 48 to achieve thedesired yaw orientation.

Monitoring module 88 may include direct sensing capabilities such asvibration sensors, voltage sensors, current sensors, strain gauges aswell as others. Monitoring module 88 may also include indirect sensingcapabilities that monitor parameters such as altitude or temperature andthen activate or modify operating parameters of tail rotors 42, 44, 46,48 manually or automatically based on the aircraft flight parameter orcondition using predefined characteristics demonstrated duringdevelopment. Monitoring module 88 includes a noise monitoring module 68a configured to monitor one or more flight parameters of rotorcraft 10.A quiet mode command module 68 b of command module 86 is configured toselectively switch tail rotors 42, 44, 46, 48 to a quiet mode based onthe one or more flight parameters monitored by noise monitoring module68 a. Quiet mode command module 68 b is also configured to modifycertain operating parameters of tail rotors 42, 44, 46, 48 in the quietmode to reduce noise emissions from rotorcraft 10. For example, quietmode command module 68 b may modify the rotational speed, angularacceleration, angular deceleration or maximum rotational speed of anycombination of tail rotors 42, 44, 46, 48 with predefined values. Inembodiments in which one or more tail rotors 42, 44, 46, 48 have areversible rotational direction, quiet mode command module 68 b may alsocontrol a motor reversal set point of the motors of such tail rotorsusing predefined values. Monitoring module 88 includes an airframeprotection monitoring module 70 a configured to monitor certain flightparameters of rotorcraft 10. An airframe protection command module 70 bof command module 86 is configured to modify certain operatingparameters of tail rotors 42, 44, 46, 48 based on the flight parametersmonitored by airframe protection monitoring module 70 a to protect theairframe of rotorcraft 10. Monitoring module 88 includes a powermanagement monitoring module 72 a configured to monitor certain flightparameters of rotorcraft 10. A power management command module 72 b ofcommand module 86 is configured to allocate power between power system58 and tail rotors 42, 44, 46, 48 based on the flight parametersmonitored by power management monitoring module 72 a. Monitoring module88 also includes a tail rotor balancing monitoring module 74 aconfigured to monitor certain parameters of rotorcraft 10 and identify afirst set of one or more tail rotors amongst tail rotors 42, 44, 46, 48based on the monitored parameters. A tail rotor balancing command module74 b of command module 86 is configured to modify certain operatingparameters of the tail rotor(s) identified by tail rotor balancingmonitoring module 74 a.

Some or all of the autonomous control capability of flight controlcomputer 56 can be augmented or supplanted by remote flight controlfrom, for example, remote system 80. Remote system 80 may include one ormore computing systems that may be implemented on general-purposecomputers, special purpose computers or other machines with memory andprocessing capability. Remote system 80 may be a microprocessor-basedsystem operable to execute program code in the form ofmachine-executable instructions. In addition, remote system 80 may beconnected to other computer systems via a proprietary encrypted network,a public encrypted network, the Internet or other suitable communicationnetwork that may include both wired and wireless connections. Remotesystem 80 communicates with flight control computer 56 via communicationlink 84 that may include both wired and wireless connections.

While operating remote control application 80 b, remote system 80 isconfigured to display information relating to one or more aircraft ofthe present disclosure on one or more flight data display devices 80 c.Remote system 80 may also include audio output and input devices such asa microphone, speakers and/or an audio port allowing an operator tocommunicate with other operators, a base station and/or a pilot onboardrotorcraft 10. Display device 80 c may also serve as a remote inputdevice 80 d if a touch screen display implementation is used, althoughother remote input devices such as a keyboard or joystick mayalternatively be used to allow an operator to provide control commandsto an aircraft being operated responsive to remote control.

Some or all of the autonomous and/or remote flight control of rotorcraft10 can be augmented or supplanted by onboard pilot flight control from apilot interface system 82 that includes one or more computing systemsthat communicate with flight control computer 56 via one or more wiredcommunication channels 94. Pilot system 82 preferably includes one ormore cockpit display devices 82 a configured to display information tothe pilot. Cockpit display device 82 a may be configured in any suitableform including, for example, a display panel, a dashboard display, anaugmented reality display or the like. Pilot system 82 may also includeaudio output and input devices such as a microphone, speakers and/or anaudio port allowing an onboard pilot to communicate with, for example,air traffic control. Pilot system 82 also includes a plurality of userinterface devices 82 b to allow an onboard pilot to provide controlcommands to rotorcraft 10 including, for example, a control panel withswitches or other inputs, mechanical control devices such as steeringdevices or sticks, voice control as well as other control devices.

Referring to FIGS. 5A-5B in the drawings, rotorcraft 100 including yawcontrol system 102 is schematically illustrated. Yaw control matrix 104is coupled to an aft portion of tailboom 106 of rotorcraft 100. Yawcontrol matrix 104 includes shroud 108, which forms ducts 110. Upperforward tail rotor 112, upper aft tail rotor 114, lower forward tailrotor 116 and lower aft tail rotor 118 are disposed in ducts 110 andsecured by stators 120. In the illustrated embodiment, shroud 108, ducts110 and tail rotors 112, 114, 116, 118 collectively form a generallyrhombus shape configuration, although shroud 108, ducts 110 and tailrotors 112, 114, 116, 118 may separately or collectively form any shapeconfiguration. Upper and lower forward tail rotors 112, 116 arevertically offset and upper and lower aft tail rotors 114, 118 arevertically offset. In addition, upper forward tail rotor 112 issubstantially horizontally aligned with upper aft tail rotor 114 andlower forward tail rotor 116 is substantially horizontally aligned withlower aft tail rotor 118. It will be appreciated by one of ordinaryskill in the art, however, that tail rotors 112, 114, 116, 118 may bepositioned in an infinite number of configurations relative to oneanother depending on the embodiment, and that yaw control matrix 104 mayinclude any number of ducts and tail rotors. In the illustratedembodiment, yaw control system 102 is an electrically distributed yawcontrol system in which tail rotor blades 112 a, 114 a, 116 a, 118 a arefixed pitch tail rotor blades and motors 112 b, 114 b, 116 b, 118 b arevariable rotational speed motors capable of changing revolutions perminute (RPMs). In other embodiments, however, tail rotor blades 112 a,114 a, 116 a, 118 a may be variable pitch tail rotor blades having afixed or variable rotational speed and may be electrically, mechanicallyor hydraulically driven. In yet other embodiments, one set of one ormore tail rotors 112, 114, 116, 118 may be fixed pitch, variablerotational speed tail rotors while a different set of one or more tailrotors 112, 114, 116, 118 are variable pitch, fixed rotational speedtail rotors. It will also be appreciated that tail rotor blades 112 a,114 a, 116 a, 118 a need not all be formed from the same material. Forexample, one set of one or more tail rotors 112, 114, 116, 118 may havetail rotor blades formed from a heavy or metallic material such asaluminum while a different set of one or more tail rotors 112, 114, 116,118 has tail rotor blades formed from a lighter or composite materialsuch as a carbon-based material. The thrust capability andresponsiveness of tail rotors 112, 114, 116, 118 may be customized bydifferentiating the material composition of tail rotor blades 112 a, 114a, 116 a, 118 a.

Current tail rotor systems are prone to blade stall within theiroperational envelope depending on the static blade angle of attack. Theillustrative embodiments avoid this drawback by being configurable intonumerous operational and spatial configurations such that tail rotors112, 114, 116, 118 are less prone to stall simultaneously or at all. Forexample, to reduce the potential for blade stall, one set of one or moretail rotors 112, 114, 116, 118 may be rotatable in a first rotationaldirection while a different set of one or more tail rotors 112, 114,116, 118 are rotatable in a second and opposite rotational direction. Asillustrated in FIGS. 5A-5B, tail rotors 112, 118 are clockwise tailrotors that rotate in rotational direction 122 while tail rotors 114,116 are counterclockwise tail rotors that rotate in rotational direction124, which is opposite of rotational direction 122. Any combination ofone or more tail rotors 112, 114, 116, 118 may rotate in eitherclockwise rotational direction 122 or counterclockwise rotationaldirection 124. In the illustrated embodiment, each tail rotor 112, 114,116, 118 is rotatable only in a single rotational direction indicated byarrows 122, 124. In other embodiments, however, the rotational directionof one or more tail rotors 112, 114, 116, 118 may be reversible. As bestseen in FIG. 5B, tail rotors 112, 114, 116, 118 each emit thrust in thesame anti-torque direction 126. In other embodiments, however, tailrotors 112, 114, 116, 118 may emit thrusts in opposite directions. Forexample, clockwise tail rotors 112, 118 may emit thrust in anti-torquedirection 126 while counterclockwise tail rotors 114, 116 emit thrust ina pro-torque direction. In embodiments in which the rotational directionof each tail rotor 112, 114, 116, 118 is reversible, each tail rotor112, 114, 116, 118 may selectively emit thrust in either direction.Also, the thrust magnitudes of tail rotors 112, 114, 116, 118 may beuniform or nonuniform depending on the rotational speeds of tail rotormotors 112 b, 114 b, 116 b, 118 b.

Referring to FIGS. 6A-6M in the drawings, various yaw control matriceshaving different tail rotor configurations are depicted. In FIG. 6A,shroud 130 of yaw control matrix 132 forms ducts 134 a, 134 b, 134 c,134 d and includes tail rotors 136 a, 136 b, 136 c, 136 d that aredifferent sizes. In the illustrated embodiment, upper forward duct 134a, upper forward tail rotor 136 a, lower aft duct 134 d and lower afttail rotor 136 d have a larger diameter than upper aft duct 134 b, upperaft tail rotor 136 b, lower forward duct 134 c and lower forward tailrotor 136 c. Although two larger ducts and tail rotors and two smallerducts and tail rotors are shown in the illustrated embodiment, yawcontrol matrix 132 may have any ratio of large ducts or tail rotors tosmall ducts or tail rotors such as 1:3 or 3:1. Upper forward duct 134 aand lower aft duct 134 d have the same diameter, upper forward tailrotor 136 a and lower aft tail rotor 136 d have the same diameter, upperaft duct 134 b and lower forward duct 134 c have the same diameter andupper aft tail rotor 136 b and lower forward tail rotor 136 c have thesame diameter. Although two duct diameters and two tail rotor diametersare shown in the illustrated embodiment, yaw control matrix 132 maycontain ducts and tail rotors having any number of nonuniform diameterssuch as three or four different diameters. Because tail rotors 136 a,136 d have a larger diameter than tail rotors 136 b, 136 c, the tailrotor blades of tail rotors 136 a, 136 d have a larger blade length thanthe tail rotor blades of tail rotors 136 b, 136 c. In addition, themotors of tail rotors 136 a, 136 d may be larger than the motors of tailrotors 136 b, 136 c in proportion to their respective sizes. Tail rotors136 a, 136 b, 136 c, 136 d also have nonuniform numbers of tail rotorblades. In the illustrated embodiment, tail rotor 136 a has eight tailrotor blades, tail rotor 136 b has five tail rotor blades, tail rotor136 c has four tail rotor blades and tail rotor 136 d has eight tailrotor blades. In embodiments in which tail rotors 136 a, 136 b, 136 c,136 d have nonuniform numbers of tail rotor blades, each tail rotor 136a, 136 b, 136 c, 136 d may have any desired number of tail rotor bladesto customize the thrust, rotational inertia, angular acceleration and/orother characteristics of each tail rotor blade 136 a, 136 b, 136 c, 136d. In some embodiments, tail rotors 136 a, 136 b, 136 c, 136 d may haveequidistant or nonequidistant spacing between their respective tailrotor blades. For example, one or more of tail rotors 136 a, 136 b, 136c, 136 d may have tail rotor blades with equidistant spacingtherebetween while another one or more of tail rotors 136 a, 136 b, 136c, 136 d has tail rotor blades that are nonuniformly, ornonequidistantly, spaced. Nonuniformly spaced tail rotor blades may beradially nonsymmetric. In other embodiments, all of tail rotors 136 a,136 b, 136 c, 136 d may have either equidistant or nonuniform spacingbetween their rotor blades.

In FIGS. 6B-6D, yaw control matrix 140 includes tail rotors 142 a, 142b, 142 c, 142 d having tail rotor blades 144 a, 144 b, 144 c, 144 d withnonuniform blade twists. In the illustrated embodiment, tail rotorblades 144 b of tail rotor 142 b have little or no twist while tailrotor blades 144 d of tail rotor 142 d have a larger blade twist thantail rotor blades 144 b of tail rotor 142 b. Tail rotor blades 144 a,144 c of tail rotors 142 a, 142 c may have little or no blade twist asin tail rotor 142 b, a larger blade twist as in tail rotor 142 d ordifferent blade twists altogether. Varying the blade twists of tailrotors 142 a, 142 b, 142 c, 142 d allows for customization of thrustefficiency, aerodynamic response and other characteristics of each tailrotor 142 a, 142 b, 142 c, 142 d. In FIGS. 6E-6G, yaw control matrix 148includes tail rotors 150 a, 150 b, 150 c, 150 d having fixed pitch tailrotor blades 152 a, 152 b, 152 c, 152 d with nonuniform pitches, orangles of attack. In the illustrated embodiment, tail rotor blades 152 bof tail rotor 150 b have a fixed pitch 154 that is smaller than fixedpitch 156 of tail rotor blades 152 d of tail rotor 150 d. Tail rotorblades 152 a, 152 c of tail rotors 150 a, 150 c may have fixed pitch 154as in tail rotor 150 b, fixed pitch 156 as in tail rotor 150 d ordifferent blade pitches altogether. In yet other embodiments, a set ofone or more tail rotors 150 a, 150 b, 150 c, 150 d may have variablepitch tail rotor blades while the remaining tail rotors 150 a, 150 b,150 c, 150 d have fixed pitch tail rotor blades. Varying the fixed pitchof tail rotor blades 152 a, 152 b, 152 c, 152 d of each tail rotor 150a, 150 b, 150 c, 150 d allows for the customization of thrustcharacteristics for each tail rotor 150 a, 150 b, 150 c, 150 d.

In FIGS. 6H-6J, yaw control matrix 160 includes tail rotors 162 a, 162b, 162 c, 162 d having tail rotor blades 164 a, 164 b, 164 c, 164 d withnonuniform airfoil shapes. In the illustrated embodiment, tail rotorblades 164 b of tail rotor 162 b have a substantially symmetricalcross-sectional airfoil shape while tail rotor blades 164 d of tailrotor 162 d have a substantially nonsymmetrical cross-sectional airfoilshape. Tail rotor blades 164 a of tail rotor 162 a and tail rotor blades164 c of tail rotor 162 c may have a symmetrical airfoil shape as intail rotor 162 b, a nonsymmetrical airfoil shape as in tail rotor 162 dor different airfoil shapes altogether. Non-limiting examples of airfoilshapes of tail rotor blades 164 a, 164 b, 164 c, 164 d includesymmetrical, symmetrical biconvex, semi-symmetrical, nonsymmetrical,nonsymmetrical biconvex, flat bottom, cambered, under-cambered, reflexcambered, supercritical or any other airfoil shapes. Tail rotor blades164 a, 164 b, 164 c, 164 d may also have a uniform airfoil shape.

In FIG. 6K, shroud 168, ducts 170 and tail rotors 172 of yaw controlmatrix 174 collectively form a generally triangular shape configuration.Yaw control matrix 174 has three tail rotors 172, thus reducing theoverall weight of yaw control matrix 174. In other embodiments, yawcontrol matrix 174 may have any number of tail rotors such as two, five,twenty, thirty or more tail rotors. In FIG. 6L, shroud 176, ducts 178and tail rotors 180 of yaw control matrix 182 collectively form agenerally hexagonal shape configuration. Yaw control matrix 182 includesseven tail rotors 180, allowing for greater redundancy in the event of atail rotor failure as well as greater variation or nonuniformity amongsttail rotors 180 themselves. In FIG. 6M, shroud 184, ducts 186 and tailrotors 188 of yaw control matrix 190 collectively form a generally arcshape configuration. Yaw control matrix 190 slopes upward such that theaft-most tail rotor is higher than the forward-most tail rotor. Yawcontrol matrix 190 includes five tail rotors 188, although any number oftail rotors may be included in yaw control matrix 190. The arc shape ofyaw control matrix 190 allows for a greater distance 192 between theforward-most and aft-most tail rotors 188, thus allowing for greatervariation between the fuselage moments exerted by each tail rotor 188.While the illustrated embodiments show several shape configurations, itwill be appreciated by one of ordinary skill in the art that theshrouds, ducts and tail rotors disclosed herein may have any shapeconfiguration such as a square, circular, parallelogram, straight-line,irregular, elliptical, polygonal or other shape configuration. Thevarious yaw control matrix configurations disclosed herein provide forredundancy in the event of loss of effectiveness of one or more of thetail rotors due to blade stall, malfunction or other factors. Multipletail rotors and customized spacing between the tail rotors may alsoprevent all tail rotors from being simultaneously within a potentiallydangerous turbulent airflow during flight of the rotorcraft.

Referring to FIGS. 7A-7D in the drawings, rotorcraft 200 is depicted asa helicopter including yaw control system 202. Yaw control matrix 204 iscoupled to an aft portion of tailboom 206 of rotorcraft 200. Yaw controlmatrix 204 includes shroud 208, which forms ducts 210. Upper forwardtail rotor 212, upper aft tail rotor 214, lower forward tail rotor 216and lower aft tail rotor 218 are disposed in ducts 210 and secured bystators 220. In the illustrated embodiment, shroud 208, ducts 210 andtail rotors 212, 214, 216, 218 collectively form a generally rhombusshape configuration, although shroud 208, ducts 210 and tail rotors 212,214, 216, 218 may separately or collectively form any shapeconfiguration. It will be appreciated by one of ordinary skill in theart that tail rotors 212, 214, 216, 218 may be positioned in an infinitenumber of configurations relative to one another depending on theembodiment, and that yaw control matrix 204 may include any number ofducts and tail rotors. In the illustrated embodiment, yaw control system202 is an electrically distributed yaw control system in which the tailrotor blades of tail rotors 212, 214, 216, 218 are fixed pitch tailrotor blades and the motors of tail rotors 212, 214, 216, 218 arevariable rotational speed motors capable of changing RPMs. In otherembodiments, however, the tail rotor blades of tail rotors 212, 214,216, 218 may be variable pitch tail rotor blades having a fixed orvariable rotational speed and may be electrically, mechanically orhydraulically driven. In yet other embodiments, one set of one or moretail rotors 212, 214, 216, 218 may be fixed pitch, variable rotationalspeed tail rotors while a different set of one or more tail rotors 212,214, 216, 218 are variable pitch, fixed rotational speed tail rotors orvariable pitch, variable rotational speed tail rotors.

Conventional helicopters use a tail rotor to control yaw in forwardflight. For example, some helicopter tailbooms include a fixed verticalfin positioned to provide a suitable anti-torque thrust for a givenforward airspeed. The fine tuning of anti-torque thrust in suchhelicopters is typically performed by varying blade pitch in a variablepitch tail rotor system. The noise produced by traditional tail rotors,however, can be unacceptably high, creating flyover acoustic problems.Such noise may be undesirable in a low noise environment or duringclandestine operations. Yaw control system 202 includes a rotatablevertical control surface to manage the yaw of rotorcraft 200 especiallyduring forward flight mode of rotorcraft 200. In particular, a rudder222 is rotatably coupled to the aft edge of shroud 208 via hinge joint224. In other embodiments, rudder 222 may be located above, below orforward of yaw control matrix 204. Hinge joint 224 is canted relative toa vertical reference axis, although in other embodiments hinge joint 224may be substantially vertical. A rudder actuator 226 is configured torotate rudder 222 about hinge joint 224.

Yaw control system 202 includes yaw controller 228 implemented by flightcontrol computer 230. Yaw controller 228 is in wired or wirelesselectrical communication with rudder actuator 226 and tail rotors 212,214, 216, 218. Rudder actuator 226 rotates rudder 222 in response tocommands from yaw controller 228. Rudder actuator 226 may mechanically,electrically or hydraulically actuate rudder 222. In embodiments inwhich rudder 222 is electrically actuated, rudder actuator 226 may useaircraft power (e.g., 28 VDC) and/or the same power source as tailrotors 212, 214, 216, 218. Rudder 222 may also be rotated usingmechanical linkages and other members interconnecting rudder 222 toanti-torque controls such as pedals. In other embodiments, yawcontroller 228 may be in mechanical or hydraulic communication withrudder 222 and tail rotors 212, 214, 216, 218. Yaw controller 228 allowsfor manual control of tail rotors 212, 214, 216, 218 and rudder 222and/or autonomous control of tail rotors 212, 214, 216, 218 and rudder222 based on the flight mode, forward airspeed, altitude, airtemperature, maneuver, operation or other flight parameters ofrotorcraft 200. Yaw controller 228 includes a tail rotor rotationalspeed reduction module 232 and a rudder control module 234, which workin conjunction with one another to control tail rotors 212, 214, 216,218 and rudder 222. Tail rotor rotational speed reduction module 232selectively switches tail rotors 212, 214, 216, 218 into a rotationalspeed reduction mode while rotorcraft 200 is in forward flight mode.Rotational speed reduction mode may be activated by tail rotorrotational speed reduction module 232 when rotorcraft 200 is in forwardflight mode or when forward airspeed 236 of rotorcraft 200 exceeds aforward airspeed threshold 238. In other examples, rotational speedreduction mode may be activated by tail rotor rotational speed reductionmodule 232 when aircraft altitude and/or ambient air temperature exceedsor falls below a predetermined threshold. When rotational speedreduction mode is activated, tail rotor rotational speed reductionmodule 232 reduces the rotational speed of one or more tail rotors 212,214, 216, 218. For example, tail rotor rotational speed reduction module232 may reduce the rotational speeds of all tail rotors 212, 214, 216,218 in the rotational speed reduction mode. In some embodiments, tailrotor rotational speed reduction module 232 may turn off, or shut down,at least one or all of tail rotors 212, 214, 216, 218 in the rotationalspeed reduction mode.

Tail rotor rotational speed reduction module 232 includes quiet modecontroller 240, which selectively switches tail rotors 212, 214, 216,218 into a quiet mode when rotorcraft 200 is in forward flight modeand/or exceeds forward airspeed threshold 238. As described in theillustrative embodiments, quiet mode controller 240 controls variousoperating parameters of tail rotors 212, 214, 216, 218 such asrotational speed and angular acceleration to lower the noise emissionsof yaw control matrix 204. Tail rotor rotational speed reduction module232 may also switch tail rotors 212, 214, 216, 218 into rotational speedreduction mode based on manual input from an operator of rotorcraft 200or from elsewhere. For example, tail rotor rotational speed reductionmodule 232 may turn off or slow down tail rotors 212, 214, 216, 218 inresponse to the pilot of rotorcraft 200 engaging a button or switch incockpit 242 of rotorcraft 200. In embodiments in which tail rotors 212,214, 216, 218 are mechanically driven using one or more driveshafts,tail rotors 212, 214, 216, 218 may be slowed down or shut off using aclutch assembly.

Rudder control module 234 controls the yaw of rotorcraft 200 by rotatingrudder 222 when tail rotors 212, 214, 216, 218 are in rotational speedreduction mode. In some embodiments, rudder control module 234 mayrotate rudder 222 based on forward airspeed 236, ambient air temperatureand/or the altitude of rotorcraft 200. For example, yaw controller 228may include a yaw change determination module 244 that determines a yawadjustment based on forward airspeed 236 of rotorcraft 200. In thisnon-limiting example, rudder control module 234 rotates rudder 222 basedon the yaw adjustment determined by yaw change determination module 244.In another non-limiting example, yaw change determination module 244 maydetermine an amount by which to change or correct the yaw of rotorcraft200 based on direct measurement. In determining the yaw adjustment forrotorcraft 200 based on direct measurement, yaw change determinationmodule 244 may include or utilize one or more sensors such as a yaw ratesensor, a yaw position sensor and/or one or more accelerometers. Ruddercontrol module 234 may then modify the yaw of rotorcraft 200 usingrudder 222 based on the direct measurements from yaw changedetermination module 244. Rudder control module 234 may also determinethe magnitude of anti-torque or pro-torque thrust that is required toachieve the desired yaw of rotorcraft 200 as determined by yaw changedetermination module 244. Rudder control module 234 may determine howquickly anti-torque or pro-torque thrust must be initiated so that thedesired yaw is achieved in a timely manner. Rudder control module 234may thus determine whether, how far and how quickly to rotate rudder222. Rudder control module 234 may also rotate rudder 222 based onmanual input from an operator of rotorcraft 200. For example, ruddercontrol module 234 may rotate rudder 222 based on pedal input frompedals 246 in cockpit 242. Any type of manual input may be used tocontrol rudder 222 such as a joystick, voice control, one or moreswitches or any other interface type. Using yaw controller 228, noiseemissions from yaw control matrix 204 are reduced or eliminated sincerudder 222 and not tail rotors 212, 214, 216, 218 are relied upon formost or all yaw control in forward flight. Yaw control system 202 isalso more energy efficient than traditional tail rotors since rudder 222consumes less power than a traditional tail rotor.

FIGS. 7B-7D show various operational scenarios of yaw control system202. In FIG. 7B, rotorcraft 200 is in forward flight mode and hasforward airspeed 236. Tail rotor rotational speed reduction module 232has slowed down or shut off tail rotors 212, 214, 216, 218 in responseto receiving manual input from an operator of rotorcraft 200 or inresponse to forward airspeed 236 exceeding forward airspeed threshold238. Rudder control module 234 has rotated rudder 222 by an angle θ₁relative to the longitudinal axis of rotorcraft 200 to produceanti-torque thrust 248 a, which counteracts the torque on the fuselageof rotorcraft 200 caused by the main rotor. Angle θ₁ by which rudder 222is rotated may be determined based on the yaw adjustment calculated byyaw change determination module 244. The magnitude of angle θ₁ may alsobe determined based on manual input such as pedal input received from anoperator of rotorcraft 200. In some embodiments, rudder 222 may beautomated to rotate to a position that maintains a proper yaworientation as described above while fine-tuning of the rudder positionis provided manually by a pilot via pedals 246. For example, rudder 222may be controlled by pedals 246 above a given airspeed to provideanti-torque fine tuning for rotorcraft 200. In some non-limitingexamples, rudder 222 may be only slightly rotated to one side in forwardflight to keep rotorcraft 200 properly aligned.

In FIG. 7C, rudder 222 has been rotated in the opposite direction byangle θ₂ relative to the longitudinal axis of rotorcraft 200 to providepro-torque thrust 248 b for rotorcraft 200. Pro-torque thrust 248 b maysometimes be desired during certain maneuvers such as quick turns. Anymagnitude of pro-torque thrust 248 b may be obtained by varying angle θ₂of rudder 222. Angle θ₂ of rudder 222 may be at or near zero such thatrudder 222 is substantially aligned with shroud 208 when rotorcraft 200is in hover mode. In FIG. 7D, rotorcraft 200 is performing a rightsideward flight maneuver in a sideward flight mode. Rudder controlmodule 234 has rotated rudder 222 such that rudder 222 forms anapproximately 90 degree angle θ₃ with the longitudinal axis ofrotorcraft 200. Rudder 222 is rotated opposite of direction of flight248 c of rotorcraft 200 to tuck rudder 222 out of the way and reduce theoverall drag experienced by rotorcraft 200 during sideward flight.Rudder 222 may also be positioned orthogonally relative to thelongitudinal axis of rotorcraft 200 when rotorcraft 200 experiencesstrong crosswinds 248 d.

Referring to FIGS. 8A-8C in the drawings, a yaw control system for arotorcraft is schematically illustrated and generally designated 250.Yaw control system 250 utilizes a yaw controller such as yaw controller228 in FIG. 7A and has multiple rudders to control the yaw of therotorcraft. In particular, yaw control system 250 includes left andright stabilizer rudders 252 a, 252 b rotatably coupled to the outboardends of horizontal stabilizer 252 c, which is fixedly coupled to thetailboom of the rotorcraft. Yaw control system 250 also controls yawusing rudder 254. Rudder 254 is rotatably coupled to shroud 256 viavertical fin 258. Vertical fin 258 is fixedly coupled to the top side ofshroud 256. In other embodiments, vertical fin 258 may be rotatablycoupled to the top side of shroud 256 such that vertical fin 258 acts asa rotatable rudder and/or replaces rudder 254. Any percentage ofvertical fin 258 may be rotatable such as 10, 25, 50, 75, or 100percent, with the remaining portion of vertical fin 258 being fixed. Inyet other embodiments, rudder 254 may be rotatably coupled to a verticalfin on the forward, bottom or aft sides of shroud 256. In operation,rudder control module 234 may rotate stabilizer rudders 252 a, 252 band/or rudder 254 in either direction to provide anti-torque thrust 260a or pro-torque thrust 260 b for the rotorcraft.

Referring to FIGS. 9A-9B in the drawings, rotorcraft 264 includes a yawcontrol matrix 266. Instead of relying on a rudder appended to yawcontrol matrix 266, the entire yaw control matrix 266 acts as arotatable vertical control surface and is rotatably coupled to the aftend of tailboom 268. In the illustrated embodiment, rudder controlmodule 234 may rotate the entirety of yaw control matrix 266 in eitherdirection to provide anti-torque thrust 270 a or pro-torque thrust 270 bfor rotorcraft 264. Yaw control matrix 266 may be rotated while the tailrotors of yaw control matrix 266 are either slowed down or shut off. Insome embodiments, one or more rudders may be rotatably coupled to yawcontrol matrix 266. In other embodiments, the tail rotors of yaw controlmatrix 266 may be used to provide additional orientational control whileyaw control matrix 266 is rotated in either direction away from thelongitudinal axis of rotorcraft 264.

Referring to FIGS. 10A-10B in the drawings, various yaw control matricesand rudders are schematically illustrated. FIGS. 10A-10B illustrate thewide variety of yaw control matrix configurations to which a rudder ofthe illustrative embodiments may be rotatably coupled for yaw control.In FIG. 10A, yaw control matrix 272 has a substantially triangular shapeconfiguration and rudder 274 is rotatably coupled to the aft edge of yawcontrol matrix 272 using a vertical and noncanted hinge joint. In FIG.10B, yaw control matrix 276 has an arc shape configuration to whichupper and lower rudders 278 a, 278 b are rotatably coupled. Rudders 278a, 278 b are coupled to the aft edge of yaw control matrix 276 usingvertical hinge joints, although canted hinge joints may also beemployed. Larger lower rudder 278 b may be used to meet most of theanti-torque demand of the aircraft while smaller upper rudder 278 a,which is more responsive, may be used to generate smaller and quickeranti-torque correction to stabilize yaw in forward flight. The ruddersof the illustrative embodiments may be rotatably coupled to any portionof a yaw control matrix using any number of hinges. Additionally, anynumber of rudders may be rotatably coupled to the yaw control matricesdisclosed herein.

Referring to FIGS. 11A-11B in the drawings, various methods forcontrolling the yaw of a rotorcraft such as a helicopter are depicted.In FIG. 11A, the method includes flying a rotorcraft in forward flightmode (step 280). The method also includes reducing the rotational speedof one or more tail rotors (step 282). Step 282 may include shuttingdown one or more tail rotors or switching one or more tail rotors intoquiet mode. The method also includes rotating a rudder to control theyaw of the rotorcraft (step 284). In FIG. 11B, the method includesflying a rotorcraft in forward flight mode (step 286). The methoddetermines whether the forward airspeed of the rotorcraft exceeds aforward airspeed threshold (step 288). If the method determines that theforward airspeed of the rotorcraft does not exceed the forward airspeedthreshold, the method loops back to step 288. If the method determinesthat the forward airspeed of the rotorcraft exceeds the forward airspeedthreshold, the method determines whether to automatically reduce therotational speed of one or more tail rotors (step 290). If the methoddetermines to automatically reduce the rotational speed of one or moretail rotors, the method reduces the rotational speed of one or more tailrotors (step 292). Optionally or alternatively, the method may increasetail rotor speed with forward airspeed to ensure that the rudder doesnot stall. If the method determines not to automatically reduce therotational speed of one or more tail rotors, the method determineswhether manual input is received to reduce the rotational speed of oneor more tail rotors (step 291). If the method determines that manualinput has been received to reduce the rotational speed of one or moretail rotors, the method reduces the rotational speed of one or more tailrotors (step 292). For safety reasons, the operator of the aircraft maybe prohibited from reducing the rotational speed of one or more tailrotors and/or engaging quiet mode when the aircraft is below apredetermined or calculated forward airspeed threshold. If the methoddetermines that manual input has not been received to reduce therotational speed of one or more tail rotors, the method returns to step288. The mode of step 292 to reduce the rotational speed of one or moretail rotors may automatically disengage below a predetermined orcalculated airspeed, regardless of whether the mode was engagedautomatically or manually. The method then determines whether manualinput has been received to rotate the rudder (step 294). If the methoddetermines that manual input to rotate the rudder has been received, themethod skips to step 298 to rotate the rudder based on the manual inputto control the yaw of the rotorcraft. If the method determines thatmanual input to rotate the rudder has not been received, the methoddetermines whether a yaw adjustment has been determined or calculatedfor the rotorcraft (step 296). Such a yaw adjustment may be determined,for example, by yaw change determination module 244 in FIG. 7A oranother module of the rotorcraft. If the method determines that a yawadjustment has not been determined for the rotorcraft, the methodreturns to step 294. If the method determines that a yaw adjustment hasbeen determined for the rotorcraft, the method rotates the rudder basedon the determined yaw adjustment to control the yaw of the rotorcraft(step 298).

Referring to FIGS. 12A-12B in the drawings, rotorcraft 300 is depictedas a helicopter including yaw control system 302. Yaw control matrix 304is coupled to an aft portion of tailboom 306 of rotorcraft 300. Yawcontrol matrix 304 includes shroud 308, which forms ducts 310. Upperforward tail rotor 312, upper aft tail rotor 314, lower forward tailrotor 316 and lower aft tail rotor 318 are disposed in ducts 310 andsecured by stators 320. In the illustrated embodiment, shroud 308, ducts310 and tail rotors 312, 314, 316, 318 collectively form a generallyrhombus shape configuration, although shroud 308, ducts 310 and tailrotors 312, 314, 316, 318 may separately or collectively form any shapeconfiguration. It will be appreciated by one of ordinary skill in theart that tail rotors 312, 314, 316, 318 may be positioned in an infinitenumber of configurations relative to one another depending on theembodiment, and that yaw control matrix 304 may include any number ofducts and tail rotors. In the illustrated embodiment, yaw control system302 is an electrically distributed yaw control system in which the tailrotor blades of tail rotors 312, 314, 316, 318 are fixed pitch tailrotor blades and the motors of tail rotors 312, 314, 316, 318 arevariable rotational speed motors capable of changing RPMs. In otherembodiments, however, the tail rotor blades of tail rotors 312, 314,316, 318 may be variable pitch tail rotor blades having a fixed orvariable rotational speed and may be electrically, mechanically orhydraulically driven. In yet other embodiments, one set of one or moretail rotors 312, 314, 316, 318 may be fixed pitch, variable rotationalspeed tail rotors while a different set of one or more tail rotors 312,314, 316, 318 are variable pitch, fixed rotational speed tail rotors orvariable pitch, variable rotational speed tail rotors. Rotorcraft 300includes engine 322, which powers main rotor 324 and generator 326 viagearbox 328. Yaw control matrix 304 including tail rotors 312, 314, 316,318 may be powered by generator 326 and/or one or more batteries 330.The motors of tail rotors 312, 314, 316, 318 are controlled using motorcontrollers 332, which send and receive commands and other data to andfrom flight control computer 334.

Conventional tail rotors produce a significant portion of the totalnoise output of a rotorcraft. The additional noise produced byconventional tail rotors may cause the rotorcraft to exceed legal, orregulatory, noise emission limits and act as a nuisance for observers onthe ground. Tail rotor noise may also interfere with missions oroperations that require a low noise signature such as clandestine orreconnaissance missions. To address the noise issues present on currentaircraft, rotorcraft 300 includes quiet mode controller 336. Quiet modecontroller 336 may be implemented by flight control computer 334 or maybe implemented as a separate unit in communication with flight controlcomputer 334 as shown in FIG. 12A. Quiet mode controller 336 includesnoise monitoring module 338 and quiet mode command module 340, whichwork in conjunction with one another to determine when to switch yawcontrol matrix 304 to quiet mode and modify the behavior of tail rotors312, 314, 316, 318 in quiet mode to reduce the noise emitted by yawcontrol matrix 304.

Noise monitoring module 338 monitors and detects one or more flightparameters of rotorcraft 300 that may be used to trigger quiet mode.Noise monitoring module 338 monitors the flight parameters of rotorcraft300 using sensors 342. Sensors 342 may include any number or combinationof the following sensors: a temperature sensor, air density sensor,location or global positioning system sensor, noise sensor, ram airsensor, rotor downwash sensor, airspeed sensor, altitude sensor,attitude sensor, wind velocity sensor, cyclic control position sensor,collective control position sensor, roll rate sensor, yaw rate sensor,pitch rate sensor, acceleration sensor such as a normal, lateral and/orlongitudinal acceleration sensor, swashplate angle sensor, rotorflapping sensor, mechanical failure sensor, health monitoring system,descent rate sensor, traffic alert sensor or any other sensor suitableto perform the illustrative embodiments disclosed herein. In someembodiments, noise monitoring module 338 and quiet mode command module340 may compare certain flight parameters of rotorcraft 300 withcorresponding flight parameter thresholds and switch one or more tailrotors 312, 314, 316, 318 to quiet mode based on the comparison betweenthe flight parameter(s) and the flight parameter threshold(s). Forexample, noise monitoring module 338 includes an altitude monitoringmodule 344, which monitors the altitude of rotorcraft 300. Altitudemonitoring module 344 uses sensors 342 to determine the altitude ofrotorcraft 300. Quiet mode command module 340 switches one or more tailrotors 312, 314, 316, 318 to quiet mode based on the altitude ofrotorcraft 300 as determined by altitude monitoring module 344.Referring to FIGS. 13A-13B in conjunction with FIGS. 12A-12B, quiet modecommand module 340 switches one or more tail rotors 312, 314, 316, 318to quiet mode when the altitude of rotorcraft 300 falls below analtitude threshold 346. In some jurisdictions, altitude threshold 346may be a regulatory threshold 348 below which the noise emitted fromrotorcraft 300 is regulated or limited to specified noise levels.Switching into quiet mode below altitude threshold 346 reduces theamount of noise from rotorcraft 300 to which observers on the ground areexposed.

Referring back to FIGS. 12A-12B, noise monitoring module 338 alsoincludes a location monitoring module 350, which monitors the locationof rotorcraft 300. Location monitoring module 350 uses sensors 342 suchas a global positioning system sensor to determine the location ofrotorcraft 300. Quiet mode command module 340 switches one or more tailrotors 312, 314, 316, 318 to quiet mode based on the location ofrotorcraft 300 as determined by location monitoring module 350.Referring to FIGS. 13C-13D in conjunction with FIGS. 12A-12B, quiet modecommand module 340 may switch one or more tail rotors 312, 314, 316, 318into quiet mode when rotorcraft 300 enters location 352. Location 352may be an urban area or other location where noise is legally regulated.The general boundary of location 352 is a non-limiting example ofregulatory thresholds 348 which, when traversed, may trigger quiet modecommand module 340 to switch one or more tail rotors 312, 314, 316, 318to quiet mode. Restricting the noise emitted from rotorcraft 300 inurban airspace, legally restricted areas or other noise sensitivelocations reduces the number of observers exposed to unacceptable noiseemissions from rotorcraft 300.

Referring back to FIGS. 12A-12B, regulatory thresholds 348 may includeother flight or operating parameter thresholds such as decibels of noiseemission or the rotational speed of tail rotors 312, 314, 316, 318, andquiet mode command module 340 may switch one or more tail rotors 312,314, 316, 318 into quiet mode when a flight or operating parameter ofrotorcraft 300 or yaw control matrix 304 traverses any of theseregulatory thresholds 348. Other predefined thresholds, other than legalor regulatory thresholds 348, may also be compared against the flightparameters of rotorcraft 300, the operating parameters of tail rotors312, 314, 316, 318 or any sensor data detected by sensors 342 todetermine when to switch one or more tail rotors 312, 314, 316, 318 intoquiet mode. Thus, monitoring modules other than altitude monitoringmodule 344 and location monitoring module 350 may be included in noisemonitoring module 338. For example, noise monitoring module 338 maycalculate a noise signature of yaw control matrix 304 based on one ormore flight parameters of rotorcraft 300, one or more operatingparameters of yaw control matrix 304 and/or sensor data from sensors342. The noise signature may include the volume, the frequency contentand/or other characteristics of the noise emitted from tail rotors 312,314, 316, 318. Quiet mode command module 340 may then switch one or moretail rotors 312, 314, 316, 318 to quiet mode based on the noisesignature calculated by noise monitoring module 338. For example, quietmode command module 340 may switch one or more tail rotors 312, 314,316, 318 to quiet mode in response to the noise emission decibel levelof yaw control matrix 304 exceeding a decibel threshold. The emission ofundesired frequencies such as undesired excitation or nuisancefrequencies may also be used by quiet mode command module 340 todetermine when to activate the quiet mode. Other parameters that may bemonitored by noise monitoring module 338 include the airspeed ofrotorcraft 300, the ambient temperature during flight, the ambient airdensity during flight, the RPMs of the motors of tail rotors 312, 314,316, 318, the blade pitch of the tail rotor blades of tail rotors 312,314, 316, 318 or any sensor data from sensors 342, and quiet modecommand module 340 may switch one or more tail rotors 312, 314, 316, 318to the quiet mode based on any of these parameters. In yet otherembodiments, the noise emitted by yaw control matrix 304 may be directlymeasured using one or more microphones or vibration sensors, and quietmode command module 340 may switch one or more tail rotors 312, 314,316, 318 into quiet mode based on these direct measurements.

The flight parameters monitored by noise monitoring module 338 may alsoinclude manual input 354 from an operator of rotorcraft 300 such as apilot, an aircraft occupant or a remote operator. In such embodiments,quiet mode command module 340 may switch one or more tail rotors 312,314, 316, 318 to quiet mode based on manual input 354. For example, thepilot of rotorcraft 300 may become aware of the need to reduce the noiseemitted by rotorcraft 300 such as in low altitude flight or in theproximity of a hostile party, in which case the pilot may send manualinput 354 to cause yaw control matrix 304 to switch into quiet mode.Thus, the quiet mode may be activated either automatically or manually.In embodiments in which the quiet mode may be automatically activated,the operator of rotorcraft 300 may be alerted when rotorcraft 300 is inquiet mode, and the quiet mode may be overridden or deactivated by theoperator if desired. The quiet mode may be activated for rotorcraft 300during any flight mode or maneuver of rotorcraft 300 including forwardflight mode or hover mode.

In quiet mode, quiet mode command module 340 modifies one or moreoperating parameters of one or more tail rotors 312, 314, 316, 318 toreduce the noise emitted by yaw control matrix 304. Quiet mode commandmodule 340 includes a rotational speed regulator 356 to modify therotational speed(s) of one or more tail rotors 312, 314, 316, 318 inquiet mode. For example, rotational speed regulator 356 may reduce therotational speed(s) of one or more tail rotors 312, 314, 316, 318 inquiet mode. Rotational speed regulator 356 may also set or modify amaximum rotational speed 358 of one or more tail rotors 312, 314, 316,318, thus limiting the rotational speed(s) of one or more tail rotors312, 314, 316, 318 to maximum rotational speed 358 in quiet mode.Modifying maximum rotational speed 358 may include changing maximumrotational speed 358 from an infinite value to a lower value, thuseffectively providing a maximum rotational speed for one or more tailrotors 312, 314, 316, 318 where none existed before.

In some embodiments, quiet mode command module 340 may also include anacceleration regulator 360 that sets or modifies a maximum angularacceleration or maximum angular deceleration for one or more tail rotors312, 314, 316, 318, thus limiting the angular acceleration or angulardeceleration of one or more tail rotors 312, 314, 316, 318 to themaximum angular acceleration or maximum angular deceleration in quietmode. For example, if acceleration regulator 360 sets a maximum angularacceleration for one or more tail rotors 312, 314, 316, 318 to a valueN, then the angular acceleration of the affected tail rotor(s) islimited to values of N and below in quiet mode. Conversely, ifacceleration regulator 360 sets a maximum angular deceleration for oneor more tail rotors 312, 314, 316, 318 to a value −N, then the angulardeceleration of the affected tail rotor(s) is limited to values of −Nand above in quiet mode. Modification of the maximum angularacceleration or maximum angular deceleration may include, in someinstances, changing the maximum angular acceleration or maximum angulardeceleration from a positive infinite value to a lower value or anegative infinite value to higher value, thus effectively providing amaximum angular acceleration or maximum angular deceleration where noneexisted before.

In some embodiments, one or more tail rotors 312, 314, 316, 318 may berotatable in either rotational direction to provide anti-torque orpro-torque thrust based on the needs of rotorcraft 300. Such reversibletail rotors reverse rotational direction at a motor reversal set point.Depending on the embodiment, the motor reversal set point may be set asa relation between pedal position and tail rotor motor speed, or mayalso be based on total anti-torque or yaw rate. Reversing rotationaldirection, however, may be a noisy operation since the tail rotor passesthrough a large range of rotational speeds by slowing down, momentarilystopping and speeding up again. Quiet mode command module 340 maycontrol when and how quickly each tail rotor 312, 314, 316, 318 changesrotational direction so as to reduce overall noise, especially in flightconditions in which noise is least acceptable. In particular, quiet modecommand module 340 may include a motor reversal set point regulator 362to set or modify the motor reversal set point(s) of one or more tailrotors 312, 314, 316, 318 in quiet mode. By changing the motor reversalset point(s) of one or more tail rotors 312, 314, 316, 318, motorreversal set point regulator 362 may cause one or more tail rotors 312,314, 316, 318 to change rotational direction less readily in quiet mode,thereby reducing the noise emissions of yaw control matrix 304. Motorreversal set point regulator 362 may be utilized during any flight modeor maneuver of rotorcraft 300. Quiet mode command module 340 may alsohave the capacity to lower transmission and/or generator noise bytemporarily increasing battery power consumption by yaw control matrix304. In particular, quiet mode command module 340 may include a powerregulation module 364 to reduce the power supplied by generator 326 toone or more tail rotors 312, 314, 316, 318 and increase the powersupplied by battery 330 to one or more tail rotors 312, 314, 316, 318 inquiet mode.

Quiet mode command module 340 may, in some instances, nonuniformlymodify operating parameters such as rotational speed, angularacceleration and power consumption of tail rotors 312, 314, 316, 318based on the size, the position or other characteristics of each tailrotor 312, 314, 316, 318. For example, larger and louder motors of tailrotors 312, 314, 316, 318 may be more strictly limited in quiet modethan smaller motors. Also, because motors of different sizes havedifferent noise signature frequencies, quiet mode command module 340 mayregulate certain frequency emissions from yaw control matrix 304 such asthose that are capable of traveling longer distances. In this example,the motors of tail rotors 312, 314, 316, 318 that emit frequencies withlonger travel distances may be more strictly limited in quiet mode thanother motors. As a further example, the position of each tail rotor 312,314, 316, 318 on shroud 308 is a factor in determining noise signature.Thus, each tail rotor 312, 314, 316, 318 may be regulated differently byquiet mode command module 340 based on the position of each tail rotor312, 314, 316, 318. In one example, the tail rotors that have thelargest effect on helicopter torque such as the aft-most tail rotor maybe used in quiet mode to full capacity while quiet mode command module340 regulates the operating parameters of the tail rotors that have theleast effect on helicopter torque such as the forward-most tail rotor.Also, smaller tail rotors may be shut down or more highly regulated byquiet mode command module 340 than larger tail rotors. The flight modeof rotorcraft 300 may also determine how each tail rotor 312, 314, 316,318 is regulated by quiet mode command module 340. For example, eachtail rotor 312, 314, 316, 318 may be regulated differently in hover modethan in forward flight mode.

The activation, deactivation and/or extent of quiet mode may also beaffected by the maneuver being performed by rotorcraft 300. Quiet modecontroller 336 may include a maneuver detection module 366 to detect themaneuver being performed by rotorcraft 300. Quiet mode command module340 may modify one or more operating parameters of one or more tailrotors 312, 314, 316, 318 in quiet mode based at least partially on themaneuver detected by maneuver detection module 366. For example,maneuvers that require a large amount of thrust from yaw control matrix304 such as sideward flight may attenuate the amount of noise reductionimplemented by quiet mode command module 340 until after the maneuver iscomplete. Maneuver detection module 366 may detect the maneuver beingperformed by rotorcraft 300 using sensor data from sensors 342. In otherembodiments, the maneuver being performed by rotorcraft 300 may bedetermined based on the power consumption of each tail rotor 312, 314,316, 318. The maneuver may also be manually inputted by a pilot or fromelsewhere.

Referring to FIGS. 14A-14B in the drawings, isometric views of anelectric vertical takeoff and landing (eVTOL) aircraft 370 for use withquiet mode controller 336 are depicted. FIG. 14A depicts eVTOL aircraft370 in a forward flight mode wherein the rotor systems provide forwardthrust with the forward airspeed of eVTOL aircraft 370 providingwing-borne lift enabling eVTOL aircraft 370 to have a high speed and/orhigh endurance forward flight mode. FIG. 14B depicts eVTOL aircraft 370in a VTOL flight mode wherein the rotor systems provide thrust-bornelift. VTOL flight mode includes takeoff, hover and landing phases offlight. In the illustrated embodiment, eVTOL aircraft 370 includes wings372 a, 372 b. Wings 372 a, 372 b have an airfoil cross-section thatgenerates lift responsive to the forward airspeed of eVTOL aircraft 370.

In the illustrated embodiment, eVTOL aircraft 370 includes four rotorsystems forming a two-dimensional distributed thrust array. The thrustarray of eVTOL aircraft 370 includes a forward-port rotor system 374 a,a forward-starboard rotor system 374 b, an aft-port rotor system 374 cand an aft-starboard rotor system 374 d, which may be referred tocollectively as rotor systems 374. Forward-port rotor system 374 a andforward-starboard rotor system 374 b are each rotatably mounted to ashoulder portion of the fuselage at a forward station thereof. Aft-portrotor system 374 c is rotatably mounted on the outboard end of wing 372a. Aft-starboard rotor system 374 d is rotatably mounted on the outboardend of wing 372 b. In the illustrated embodiment, rotor systems 374 areducted rotor systems each having a five bladed rotor assembly withvariable pitch rotor blades operable for collective pitch control. Rotorsystems 374 may each include at least one variable speed electric motorand a speed controller configured to provide variable speed control tothe rotor assembly over a wide range of rotor speeds, or alternativelymay each include at least one constant speed electric motor to providefixed RPM. In other embodiments, the rotor systems could be non-ductedor open rotor systems, the number of rotor blades could be eithergreater than or less than five and/or the rotor blades could have afixed pitch. eVTOL aircraft 370 may include any number of rotor systemseither greater than or less than four rotor systems such as a coaxialrotor system or six rotor systems.

When eVTOL aircraft 370 is operating in the forward flight orientationand supported by wing-borne lift, rotor systems 374 each have agenerally vertical position with the forward rotor assemblies rotatinggenerally in a forward vertical plane and the aft rotor assembliesrotating generally in an aft vertical plane, as best seen in FIG. 14A.When eVTOL aircraft 370 is operating in the VTOL orientation andsupported by thrust-borne lift, rotor systems 374 each have a generallyhorizontal position such that the rotor assemblies are rotating ingenerally the same horizontal plane, as best seen in FIG. 14B.Transitions between the VTOL orientation and the forward flightorientation of eVTOL aircraft 370 are achieved by changing the angularpositions of rotor systems 374 between their generally horizontalpositions and their generally vertical positions. Quiet mode controller336 in FIG. 12B may be used to control the noise emissions of rotorsystems 374 in a similar manner to that described for yaw control matrix304. For example, noise monitoring module 338 may monitor one or moreparameters of eVTOL aircraft 370 and quiet mode command module 340 maymodify one or more operating parameters of rotor systems 374 based onthe parameters monitored and detected by noise monitoring module 338.Thus, noise controller 336 is not limited to being used for yaw controlsystems only and may be used for a wide variety of rotor propulsionsystems utilized on rotorcraft.

Referring to FIG. 15 in the drawings, a method for managing the noiseemissions of one or more tail rotors of a rotorcraft such as ahelicopter is depicted. The method includes detecting one or more flightparameters of a rotorcraft (step 378). The method also includesswitching one or more tail rotors of the rotorcraft to a quiet modebased on the one or more flight parameters (step 380). The method alsoincludes modifying one or more operating parameters of the one or moretail rotors in quiet mode to reduce the noise emitted by the rotorcraft(step 382). In some embodiments, the method may include calculating anoise signature of one or more tail rotors based on one or more flightparameters of the rotorcraft and/or one or more operating parameters ofthe tail rotors. Such a method may include switching one or more tailrotors to quiet mode in response to the noise signature exceeding anoise signature threshold. In other embodiments, the method may includecomparing one or more flight parameters with one or more flightparameter thresholds. Such a method may include switching one or moretail rotors to quiet mode based on the comparison between one or moreflight parameters and one or more flight parameter thresholds. In otherembodiments, the method may include detecting a maneuver being performedby the rotorcraft. In such a method, one or more operating parameters ofone or more tail rotors may be modified in quiet mode based at leastpartially on the detected maneuver.

Referring to FIGS. 16A-16B in the drawings, rotorcraft 400 is depictedas a helicopter including yaw control system 402. Yaw control matrix 404is coupled to an aft portion of tailboom 406 of rotorcraft 400. Yawcontrol matrix 404 includes shroud 408, which forms ducts 410. Upperforward tail rotor 412, upper aft tail rotor 414, lower forward tailrotor 416 and lower aft tail rotor 418 are disposed in ducts 410 andsecured by stators 420. In the illustrated embodiment, shroud 408, ducts410 and tail rotors 412, 414, 416, 418 collectively form a generallyrhombus shape configuration, although shroud 408, ducts 410 and tailrotors 412, 414, 416, 418 may separately or collectively form any shapeconfiguration. It will be appreciated by one of ordinary skill in theart that tail rotors 412, 414, 416, 418 may be positioned in an infinitenumber of configurations relative to one another depending on theembodiment, and that yaw control matrix 404 may include any number ofducts and tail rotors. In the illustrated embodiment, yaw control system402 is an electrically distributed yaw control system in which tailrotor blades 412 a, 414 a, 416 a, 418 a of tail rotors 412, 414, 416,418 are fixed pitch tail rotor blades and the motors of tail rotors 412,414, 416, 418 are variable rotational speed motors capable of changingRPMs. In other embodiments, however, tail rotor blades 412 a, 414 a, 416a, 418 a of tail rotors 412, 414, 416, 418 may be variable pitch tailrotor blades having a fixed or variable rotational speed and may beelectrically, mechanically or hydraulically driven. In yet otherembodiments, one set of one or more tail rotors 412, 414, 416, 418 maybe fixed pitch, variable rotational speed tail rotors while a differentset of one or more tail rotors 412, 414, 416, 418 are variable pitch,fixed rotational speed tail rotors variable pitch, variable rotationalspeed tail rotors.

Sharp or sudden anti-torque input such as pedal input from the pilot ofa helicopter can cause the tail rotor to flap out of its plane ofrotation and contact the tailboom or other portion of the helicopterairframe. For example, as shown in FIG. 16B, loads such as bending loadson tail rotor blades 412 a, 414 a, 416 a, 418 a of tail rotors 412, 414,416, 418 may cause tail rotor blades 412 a, 414 a, 416 a, 418 a to bendand potentially collide with a portion of the airframe of rotorcraft 400such as stators 420. Such a collision may cause damage to tail rotors412, 414, 416, 418 or the airframe of rotorcraft 400 such as stators420, shroud 408 or tailboom 406. To address this issue, yaw controlsystem 402 includes an airframe protection system 422, which includesairframe protection module 424 implemented by flight control computer426 of rotorcraft 400. Airframe protection system 422 protects theintegrity of the airframe of rotorcraft 400 as well as tail rotors 412,414, 416, 418 during the various flight modes, maneuvers and otherflight conditions of rotorcraft 400. For example, airframe protectionsystem 422 may prevent the pilot of rotorcraft 400 from inputting anexcessive anti-torque input at high airspeeds or during certainmaneuvers to prevent damage to the airframe of rotorcraft 400. Toprotect the airframe of rotorcraft 400, airframe protection system 422controls the behavior of tail rotors 412, 414, 416, 418 based onmonitored flight parameters of rotorcraft 400 and/or operatingparameters of tail rotors 412, 414, 416, 418. Airframe protection module424 includes an airframe protection monitoring module 428 and anairframe protection command module 430, which work in conjunction withone another to monitor one or more parameters of rotorcraft 400 andmodify one or more operating parameters of one or more tail rotors 412,414, 416, 418 based on the one or more monitored parameters ofrotorcraft 400, thereby protecting the airframe of rotorcraft 400.

Airframe protection monitoring module 428 includes an airspeedmonitoring module 432 to monitor the airspeed of rotorcraft 400.Airframe protection command module 430 modifies one or more operatingparameters of one or more tail rotors 412, 414, 416, 418 based on theairspeed of rotorcraft 400 detected by airspeed monitoring module 432.Referring to FIGS. 17A-17B in conjunction with FIGS. 16A-16B, rotorcraft400 is shown traveling at a low airspeed 434 a and a higher airspeed 434b. Airframe protection command module 430 may modify one or moreoperating parameters of one or more tail rotors 412, 414, 416, 418 inresponse to the airspeed of rotorcraft 400 exceeding an airspeedthreshold 436. For example, airframe protection command module 430 maymodify one or more operating parameters of one or more tail rotors 412,414, 416, 418 when rotorcraft 400 is traveling at airspeed 434 b asshown in FIG. 17B if airspeed 434 b exceeds airspeed threshold 436.Controlling the behavior of tail rotors 412, 414, 416, 418 at highairspeeds 434 b is particularly beneficial since tail rotor blades 412a, 414 a, 416 a, 418 a may be more susceptible to bending loads whenrotorcraft 400 is traveling at higher speeds. In certain embodiments,however, the operating parameters of tail rotors 412, 414, 416, 418 maybe controlled at lower airspeeds 434 a or during hover mode since tailrotor blades 412 a, 414 a, 416 a, 418 a may in some cases be susceptibleto bending loads at lower airspeeds 434 a as well.

Returning to FIGS. 16A-16B, airframe protection monitoring module 428may include maneuver detection module 438, which detects the maneuverbeing performed by rotorcraft 400. Airframe protection command module430 modifies one or more operating parameters of one or more tail rotors412, 414, 416, 418 based on the maneuver detected by maneuver detectionmodule 438. Maneuver detection module 438 may detect the maneuver beingperformed by rotorcraft 400 using sensor data from on-board sensors. Inother embodiments, the maneuver being performed by rotorcraft 400 may bedetermined based on the power consumption of each tail rotor 412, 414,416, 418. The maneuver may also be manually inputted by a pilot or fromelsewhere. Referring to FIG. 17C in conjunction with FIGS. 16A-16B,rotorcraft 400 is performing a sideward flight maneuver by traveling ina sideward direction 440. Because high thrust may be required from yawcontrol matrix 404 when rotorcraft 400 performs a sideward flightmaneuver, tail rotor blades 412 a, 414 a, 416 a, 418 a may beparticularly susceptible to bending loads when performing suchmaneuvers. Airframe protection command module 430 modifies one or moreoperating parameters of one or more tail rotors 412, 414, 416, 418 inresponse to maneuver detection module 438 detecting the sideward flightmaneuver. The sideward flight maneuver performed by rotorcraft 400 inFIG. 17C is one among many maneuvers that may be detected by maneuverdetection module 438. Other non-limiting examples of maneuvers that maytrigger a change in behavior of tail rotors 412, 414, 416, 418 includefull power climbs, sharp turns or steep descents.

Returning to FIGS. 16A-16B, airframe protection monitoring module 428may include load determination module 442. Load determination module 442detects loads such as bending loads on tail rotor blades 412 a, 414 a,416 a, 418 a using one or more sensors such as strain gauges,accelerometers, Hall Effect sensors or other sensor types. Such sensorsmay be located on or near each tail rotor 412, 414, 416, 418. Airframeprotection command module 430 modifies one or more operating parametersof one or more tail rotors 412, 414, 416, 418 based on the loads on tailrotor blades 412 a, 414 a, 416 a, 418 a as detected by loaddetermination module 442. In some embodiments, airframe protectioncommand module 430 may modify one or more operating parameters of one ormore tail rotors 412, 414, 416, 418 in response to the detected load onone or more tail rotors 412, 414, 416, 418 exceeding a load threshold444. Load threshold 444 may be predetermined based on the design ofrotorcraft 400 and/or validated during flight testing. For example,flight tests may be conducted on rotorcraft 400 to determine the maximumtolerable load on tail rotor blades 412 a, 414 a, 416 a, 418 a andoperator input 446 such as anti-torque input that correlates to themaximum tolerable load. In this example, airframe protection commandmodule 430 may limit operator input 446 to values that result in theload on tail rotor blades 412 a, 414 a, 416 a, 418 a being less than themaximum tolerable load.

Load determination module 442 may also detect a load on the airframe ofrotorcraft 400 using sensors 448 such as strain gauges, accelerometers,Hall Effect sensors or other sensor types. Airframe protection commandmodule 430 may then modify one or more operating parameters of one ormore tail rotors 412, 414, 416, 418 based on the airframe load detectedby load determination module 442. In certain embodiments, airframeprotection command module 430 may modify one or more operatingparameters of one or more tail rotors 412, 414, 416, 418 in response tothe airframe load on rotorcraft 400 exceeding load threshold 444, whichmay be predetermined in flight testing or calculated based on the designof rotorcraft 400 including the anticipated load tolerance of theairframe structure. For example, flight tests may be conducted todetermine the maximum tolerable airframe load on tailboom 406. In thisexample, airframe protection command module 430 may limit operator input446 to values that result in the load on tailboom 406 being less thanthe maximum tolerable airframe load.

Airframe protection monitoring module 428 may also include a tail rotorblade clearance monitoring module 450, which detects a clearancedistance 452 between tail rotor blades 412 a, 414 a, 416 a, 418 a andthe airframe of rotorcraft 400 using sensors such as accelerometers,Hall Effect sensors or other sensor types on or near tail rotors 412,414, 416, 418. Airframe protection command module 430 modifies one ormore operating parameters of one or more tail rotors 412, 414, 416, 418based on clearance distance 452 detected by tail rotor blade clearancemonitoring module 450. For example, airframe protection command module430 may modify one or more operating parameters of one or more tailrotors 412, 414, 416, 418 in response to clearance distance 452 beingless than a minimum tail rotor blade clearance threshold 454.

Airframe protection command module 430 is capable of modifying one ormore operating parameters of one or more tail rotors 412, 414, 416, 418based on one or more flight or operating parameters detected by airframeprotection monitoring module 428 to prevent contact between tail rotorblades 412 a, 414 a, 416 a, 418 a and stators 420 or other portions ofthe airframe of rotorcraft 400. In controlling the behavior of tailrotors 412, 414, 416, 418, airframe protection command module 430 maydirectly control one or more operating parameters of tail rotors 412,414, 416, 418 such as by directly controlling the motors of tail rotors412, 414, 416, 418. Airframe protection command module 430 may alsoindirectly control the behavior of tail rotors 412, 414, 416, 418 bymodifying operator input 446 such as anti-torque input from an operatorof rotorcraft 400 based on the parameters detected by airframeprotection monitoring module 428.

Airframe protection command module 430 includes rotational speedregulator 456, which modifies the rotational speed(s) of one or moretail rotors 412, 414, 416, 418 based on the parameters monitored byairframe protection monitoring module 428. In regulating the rotationalspeeds of tail rotors 412, 414, 416, 418, rotational speed regulator 456also regulates the thrust emitted by each tail rotor 412, 414, 416, 418,thereby also limiting thrust variation. Rotational speed regulator 456may reduce the rotational speed(s) of one or more tail rotors 412, 414,416, 418 based on one or more of the parameters monitored by airframeprotection monitoring module 428. For example, rotational speedregulator 456 may reduce the rotational speed(s) of one or more tailrotors 412, 414, 416, 418 if rotorcraft 400 exceeds airspeed threshold436. Rotational speed regulator 456 may also set or modify a maximumrotational speed 458 for one or more tail rotors 412, 414, 416, 418.Rotational speed regulator 456 may thus limit the rotational speed(s) ofone or more tail rotors 412, 414, 416, 418 to maximum rotational speed458 based on one or more parameters detected by airframe protectionmonitoring module 428. For example, rotational speed regulator 456 mayset maximum rotational speed 458 for one or more tail rotors 412, 414,416, 418 if rotorcraft 400 performs a sideward flight maneuver.Modifying maximum rotational speed 458 may include changing maximumrotational speed 458 from an infinite value to a lower value, thuseffectively providing a maximum rotational speed for one or more tailrotors 412, 414, 416, 418 where none existed before.

Airframe protection command module 430 includes an accelerationregulator 460, which sets or modifies a maximum angular acceleration ora maximum angular deceleration of one or more tail rotors 412, 414, 416,418. Acceleration regulator 460 may thus limit angular acceleration orangular deceleration of one or more tail rotors 412, 414, 416, 418 tothe maximum angular acceleration or the maximum angular decelerationbased on the one or more parameters monitored by airframe protectionmonitoring module 428. For example, if the load experienced by one oftail rotors 412, 414, 416, 418 exceeds load threshold 444, airframeprotection command module 430 may limit the angular acceleration orangular deceleration for that tail rotor. If acceleration regulator 460sets a maximum angular acceleration for one or more tail rotors 412,414, 416, 418 to a value N, then the angular acceleration of theaffected tail rotor(s) may be limited to values of N and below.Conversely, if acceleration regulator 460 sets a maximum angulardeceleration for one or more tail rotors 412, 414, 416, 418 to a value−N, then the angular deceleration of the affected tail rotor(s) may belimited to values of −N and above. Modification of the maximum angularacceleration or maximum angular deceleration may include, in someinstances, changing the maximum angular acceleration or maximum angulardeceleration from a positive infinite value to a lower value or anegative infinite value to higher value, thus effectively providing amaximum angular acceleration or maximum angular deceleration where noneexisted before.

In embodiments in which one or more tail rotors 412, 414, 416, 418 havevariable pitch tail rotor blades, airframe protection command module 430may include a blade pitch regulator 462. Blade pitch regulator 462 maymodify the blade pitch of the variable pitch tail rotor blades on one ormore tail rotors 412, 414, 416, 418 based on the one or more parametersmonitored by airframe protection monitoring module 428. For example,blade pitch regulator 462 may lower the blade pitch of the tail rotorblades of one or more tail rotors 412, 414, 416, 418 at high airspeedsto reduce the loads experienced by the tail rotor blades.

Airframe protection command module 430 may, in some embodiments,uniformly modify or set the operating parameters of tail rotors 412,414, 416, 418. Airframe protection command module 430 may alsononuniformly modify the operating parameters of tail rotors 412, 414,416, 418 based on the size, position or other characteristics of eachtail rotor 412, 414, 416, 418. For example, larger motors of tail rotors412, 414, 416, 418 may be more strictly limited than smaller motors whenrotorcraft 400 exceeds airspeed threshold 436. Also, because motors ofdifferent sizes have different excitation frequencies, airframeprotection command module 430 may target certain frequency emissionsfrom yaw control matrix 404 such as those that overlap a naturalfrequency of the airframe. In this example, the motors of tail rotors412, 414, 416, 418 that emit frequencies that overlap a naturalfrequency may be more strictly limited than other motors to protect theairframe structure of rotorcraft 400. Each tail rotor 412, 414, 416, 418may also be regulated differently by airframe protection command module430 based on the position of each tail rotor 412, 414, 416, 418 in yawcontrol matrix 404. In other non-limiting examples, smaller tail rotorsmay be shut down or more highly regulated by airframe protection commandmodule 430 than larger tail rotors. The flight mode of rotorcraft 400may also determine how each tail rotor 412, 414, 416, 418 is regulatedby airframe protection command module 430. For example, each tail rotor412, 414, 416, 418 may be regulated differently in hover mode than inforward flight mode. The maneuver being performed by rotorcraft 400, asdetected by maneuver detection module 438, may also determine how eachtail rotor 412, 414, 416, 418 is regulated by airframe protectioncommand module 430. In the event that one or more tail rotors are notrotating due to, for example, being shut down to reduce noise or a motorfailure, the maximum commandable rotational speed, acceleration and/orpitch angle may be different or absent on the remaining tail rotors. Assuch, airframe protection monitoring module 428 may also determine thecurrent tail rotor configuration that is active on the aircraft.Alternatively or additionally, the cockpit may include an overrideswitch that allows the operator of the aircraft to override the airframeprotection system in some flight conditions or following an electricalmotor failure, among other operational circumstances.

Airframe protection system 422 may be used by yaw control system 402 toprevent tail rotor blades 412 a, 414 a, 416 a, 418 a from contacting theairframe of rotorcraft 400 including stators 420 in each duct 410.Airframe protection system 422 may prevent the operator of rotorcraft400 from commanding a sharp change in anti-thrust load in some flightconditions that could result in structural damage to the airframe ofrotorcraft 400 or tail rotors 412, 414, 416, 418. Because airframeprotection module 424 may be implemented by flight control computer 426,the cost and weight penalty typically associated with hardwarecomponents may be reduced or eliminated using the illustrativeembodiments.

Referring to FIGS. 18A-18B in the drawings, methods for protecting anairframe of a rotorcraft such as a helicopter are depicted. In FIG. 18A,the method includes monitoring one or more flight parameters of arotorcraft (step 466). In some embodiments, step 466 may includemonitoring at least one of an airspeed of the rotorcraft, a maneuverbeing performed by the rotorcraft, a load experienced by the rotorcraft,a clearance distance between the airframe of the rotorcraft and the tailrotor blades of one or more tail rotors and/or the status of the tailrotors (e.g., operating, shut down or failed). The method may alsoinclude modifying one or more operating parameters of one or more tailrotors of the rotorcraft based on the one or more flight parameters(step 468). Step 468 may include modifying at least one of a maximumangular acceleration, a maximum angular deceleration, a maximumrotational speed, a rotational speed or a blade pitch of one or moretail rotors. In other embodiments, step 468 may include modifyinganti-torque input from an operator of the rotorcraft based on the one ormore flight parameters. In FIG. 18B, the method includes monitoring theairspeed of a rotorcraft (step 470). The method determines whether theairspeed of the rotorcraft exceeds an airspeed threshold (step 472). Ifthe method determines that the airspeed of the rotorcraft does notexceed the airspeed threshold, the method returns to step 470. If it isdetermined that the airspeed of the rotorcraft exceeds the airspeedthreshold, the method modifies one or more operating parameters of oneor more tail rotors of the rotorcraft (step 474). The method continuesto monitor the airspeed of the rotorcraft (step 476). The methoddetermines whether the airspeed of the rotorcraft falls below theairspeed threshold (step 478). If the method determines that theairspeed of the rotorcraft has not fallen below the airspeed threshold,the method returns to step 476. If the method determines that theairspeed of the rotorcraft has fallen below the airspeed threshold, themethod reverts back to normal operating parameters of the tail rotor(s)(step 480). The method may then return to step 470.

Referring to FIG. 19 in the drawings, rotorcraft 500 is depicted as ahelicopter including yaw control system 502. Yaw control matrix 504 iscoupled to an aft portion of tailboom 506 of rotorcraft 500. Yaw controlmatrix 504 includes shroud 508, which forms ducts 510. Upper forwardtail rotor 512, upper aft tail rotor 514, lower forward tail rotor 516and lower aft tail rotor 518 are disposed in ducts 510 and secured bystators 520. In the illustrated embodiment, shroud 508, ducts 510 andtail rotors 512, 514, 516, 518 collectively form a generally rhombusshape configuration, although shroud 508, ducts 510 and tail rotors 512,514, 516, 518 may separately or collectively form any shapeconfiguration. It will be appreciated by one of ordinary skill in theart that tail rotors 512, 514, 516, 518 may be positioned in an infinitenumber of configurations relative to one another depending on theembodiment, and that yaw control matrix 504 may include any number ofducts and tail rotors. In the illustrated embodiment, yaw control system502 is an electrically distributed yaw control system in which the tailrotor blades of tail rotors 512, 514, 516, 518 are fixed pitch tailrotor blades and motors 512 a, 514 a, 516 a, 518 a of tail rotors 512,514, 516, 518 are variable rotational speed motors capable of changingRPMs. In other embodiments, however, the tail rotor blades of tailrotors 512, 514, 516, 518 may be variable pitch tail rotor blades havingan electrically-driven fixed or variable rotational speed. In yet otherembodiments, one set of one or more tail rotors 512, 514, 516, 518 maybe fixed pitch, variable rotational speed tail rotors while a differentset of one or more tail rotors 512, 514, 516, 518 are variable pitch,fixed rotational speed tail rotors.

Rotorcraft 500 includes power system 522 used to power yaw controlsystem 502. Power system 522 includes generator 524 such as a startergenerator powered by engine 526. Power system 522 also includes one ormore batteries 528, which may include any number of batteries. Forexample, battery 528 may include additional batteries to lengthen theamount of time that rotorcraft 500 can perform an operation with a highpower demand. In some embodiments, battery 528 may be used to powerelectrical aircraft equipment 530 such as cockpit displays, internallights, external lights, motor controllers as well as other electricalcomponents. Battery 528 may include a power converter to convert powerto the format used by electrical aircraft equipment 530. For example,battery 528 may incorporate a 28-volt direct current power converter. Byincorporating a power converter, generator 524 may be switched off inflight if desired, thereby improving power efficiency. In someembodiments, generator 524 may be switched off in flight while one ormore high voltage electrical generators for yaw control system 502,which may be installed on the aircraft main gearbox, remain inoperation. Battery 528 may also be used to power electrical aircraftequipment 530 during an engine loss event.

In traditional helicopters, power components such as generators andbatteries must be sized to provide peak power values at all timessimultaneously, even though such peak power values are used only inlimited circumstances such as during certain maneuvers. Rotorcraft 500includes a power management system 532 to address this issue. Powermanagement system 532 uses battery 528 in conjunction with generator 524to provide power to tail rotor motors 512 a, 514 a, 516 a, 518 a inelectrically distributed yaw control system 502. Power management system532 enables the downsizing of generator 524 so that it provides only aportion of the power required by yaw control system 502 in highdemanding flight conditions such as during hover mode. In suchconditions, battery 528 may be used to supplement the power provided bygenerator 524. When rotorcraft 500 transitions into flight conditionsthat require less power such as forward flight mode, generator 524 mayrecharge battery 528 while providing most or all of the power to tailrotor motors 512 a, 514 a, 516 a, 518 a. Power management system 532includes a power distribution unit 534, which may be a standalone unitseparate from the flight control computer of rotorcraft 500. In otherembodiments, power distribution unit 534 including power managementmodule 536 may be implemented by the flight control computer ofrotorcraft 500. Power management module 536 includes power managementmonitoring module 538, which monitors one or more flight parameters ofrotorcraft 500. Power management monitoring module 538 works inconjunction with power management command module 540, which allocatespower between power system 522 and tail rotor motors 512 a, 514 a, 516a, 518 a based on the one or more flight parameters of rotorcraft 500monitored by power management monitoring module 538.

Power management monitoring module 538 may include a flight modemonitoring module 542 to monitor and detect the flight mode ofrotorcraft 500. Power management command module 540 allocates powerbetween power system 522 and one or more tail rotor motors 512 a, 514 a,516 a, 518 a based on the flight mode detected by flight mode monitoringmodule 542. Referring to FIGS. 20A-20B in conjunction with FIG. 19 ,when flight mode monitoring module 542 detects that rotorcraft 500 is inhover mode as shown in FIG. 20A, power management command module 540 ofpower distribution unit 534 allocates power from generator 524 andbattery 528 to one or more tail rotor motors 512 a, 514 a, 516 a, 518 a.Power from both generator 524 and battery 528 may be useful or necessarywhen rotorcraft 500 is in hover mode so that yaw control matrix 504 canprovide sufficient anti-torque thrust to control the yaw of rotorcraft500. Battery 528 may continue to provide power to electrical aircraftequipment 530 when rotorcraft 500 is in hover mode.

Referring to FIGS. 20C-20D in conjunction with FIG. 19 , flight modemonitoring module 542 may also detect whether rotorcraft 500 is inforward flight mode. FIG. 20C illustrates rotorcraft 500 in forwardflight mode having a forward airspeed 544. Power management commandmodule 540 of power distribution unit 534 allocates power from generator524 to one or more tail rotor motors 512 a, 514 a, 516 a, 518 a whenrotorcraft 500 is in forward flight mode. In contrast to hover mode,however, power distribution unit 534 does not allocate power frombattery 528 to tail rotor motors 512 a, 514 a, 516 a, 518 a whenrotorcraft 500 is in forward flight mode. Power from battery 528 may beless essential when rotorcraft 500 is in forward flight mode sincerotorcraft 500 can maintain a stable yaw orientation using fins or othercontrol surfaces. In some embodiments, power management command module540 of power distribution unit 534 may allocate power from generator 524to battery 528 when rotorcraft 500 is in forward flight mode to rechargebattery 528 during this time. Thus, battery 528 may be recharged andready to provide power to tail rotor motors 512 a, 514 a, 516 a, 518 aduring other flight modes or operations of rotorcraft 500 such as hovermode. Battery 528 may continue to supply power to electrical aircraftequipment 530 when rotorcraft 500 is in forward flight mode.

Referring back to FIG. 19 , in some embodiments flight mode monitoringmodule 542 may include an airspeed monitoring module 546 to monitor theairspeed of rotorcraft 500. Power management command module 540allocates power between power system 522 and one or more tail rotormotors 512 a, 514 a, 516 a, 518 a based on the airspeed of rotorcraft500. Referring to FIGS. 20C-20D in conjunction with FIG. 19 , whenforward airspeed 544 of rotorcraft 500 exceeds airspeed threshold 544 aas shown in FIG. 20C, power management command module 540 of powerdistribution unit 534 allocates power from generator 524 to one or moretail rotor motors 512 a, 514 a, 516 a, 518 a and does not allocate anypower from battery 528 to tail rotor motors 512 a, 514 a, 516 a, 518 a.In addition, when forward airspeed 544 of rotorcraft 500 exceedsairspeed threshold 544 a, power management command module 540 of powerdistribution unit 534 may recharge battery 528 by allocating power fromgenerator 524 to battery 528. For example, power management commandmodule 540 of power distribution unit 534 may allocate power from bothgenerator 524 and battery 528 to one or more tail rotor motors 512 a,514 a, 516 a, 518 a in response to forward airspeed 544 being less thanairspeed threshold 544 a.

Returning to FIG. 19 , power management monitoring module 538 mayinclude a maneuver detection module 548 to detect the maneuver beingperformed by rotorcraft 500. Maneuver detection module 548 may detectthe maneuver being performed by rotorcraft 500 using sensor data fromon-board sensors. In other embodiments, the maneuver being performed byrotorcraft 500 may be determined based on the power consumption of eachtail rotor 512, 514, 516, 518. The maneuver may also be manuallyinputted by a pilot or from elsewhere. Power management command module540 allocates power between power system 522 and one or more tail rotormotors 512 a, 514 a, 516 a, 518 a based on the maneuver detected bymaneuver detection module 548. Whether generator 524 and/or battery 528is utilized to provide power to yaw control system 502 may depend on theamount of yaw control thrust required of yaw control matrix 504 for aparticular maneuver. Referring to FIGS. 20E-20F in conjunction with FIG.19, rotorcraft 500 is shown performing a sideward flight maneuver.Because sideward flight maneuvers generally require higher levels ofthrust from the yaw control system of a helicopter, power managementcommand module 540 of power distribution unit 534 allocates power fromboth generator 524 and battery 528 to one or more tail rotor motors 512a, 514 a, 516 a, 518 a when maneuver detection module 548 detects thatrotorcraft 500 is performing a sideward flight maneuver. Battery 528 maycontinue to provide power to electrical aircraft equipment 530 duringthe sideward flight maneuver. Power management command module 540 mayalso allocate power to yaw control matrix 504 based on other types ofmaneuvers. For example, power management command module 540 may allocatepower from both generator 524 and battery 528 to one or more tail rotormotors 512 a, 514 a, 516 a, 518 a in response to maneuver detectionmodule 548 detecting a high power or full power climb maneuver beingperformed by rotorcraft 500. Since rotorcraft 500 may not be movingforward at a fast airspeed during a high power or full power climbmaneuver, additional power may be needed by yaw control matrix 504 tocontrol the yaw of rotorcraft 500. Another maneuver that may be detectedby maneuver detection module 548 is the attachment or carrying ofexternal or internal payloads exceeding a weight threshold. Whenmaneuver detection module 548 detects such a payload, power distributionunit 534 may allocate additional power to tail rotor motors 512 a, 514a, 516 a, 518 a from battery 528 if required by yaw control matrix 504to balance yaw while carrying the payload.

Returning to FIG. 19 , power management monitoring module 538 mayinclude a flight condition monitoring module 550 to monitor a flightcondition of rotorcraft 500 during flight. For example, flight conditionmonitoring module 550 may monitor any ambient or environmental conditionaround rotorcraft 500 during flight such as crosswinds, temperature, airdensity, altitude, location or other conditions. Power managementcommand module 540 allocates power between power system 522 and one ormore tail rotor motors 512 a, 514 a, 516 a, 518 a based on the flightcondition detected by flight condition monitoring module 550. Forexample, hover at sea level versus higher altitudes may not require thesame power from yaw control matrix 504. In this example, powermanagement command module 540 of power distribution unit 534 mayallocate power from both generator 524 and battery 528 to one or moretail rotor motors 512 a, 514 a, 516 a, 518 a in response to rotorcraft500 exceeding an altitude threshold. Referring to FIGS. 20G-20H inconjunction with FIG. 19 , rotorcraft 500 is experiencing crosswinds552. Power management command module 540 of power distribution unit 534may allocate power from both generator 524 and battery 528 to one ormore tail rotor motors 512 a, 514 a, 516 a, 518 a in response tocrosswind 552 exceeding a crosswind threshold. Crosswinds 552 may have asimilar impact on rotorcraft 500 as a sideward flight maneuver andtherefore require additional power to be allocated from battery 528 toyaw control matrix 504. Battery 528 may continue to supply power toelectrical aircraft equipment 530 while rotorcraft 500 experiencescrosswinds 552.

Returning to FIG. 19 , power management monitoring module 538 mayinclude a power demand monitoring module 554, which monitors the powerdemand of one or more tail rotor motors 512 a, 514 a, 516 a, 518 a.Power management command module 540 allocates power between power system522 and yaw control matrix 504 based on the power demand of tail rotormotors 512 a, 514 a, 516 a, 518 a as detected by power demand monitoringmodule 554. For example, power management command module 540 mayallocate power from both generator 524 and battery 528 to one or moretail rotor motors 512 a, 514 a, 516 a, 518 a in response to the powerdemand of one or more tail rotor motors 512 a, 514 a, 516 a, 518 a asdetected by power demand monitoring module 554 exceeding a power demandthreshold 554 a. Conversely, power management command module 540 mayallocate power from only generator 524 to one or more tail rotor motors512 a, 514 a, 516 a, 518 a in response to the power demand of one ormore tail rotor motors 512 a, 514 a, 516 a, 518 a being less than powerdemand threshold 554 a. Power management command module 540 may alsoallocate power from generator 524 to battery 528 when the power demandof one or more tail rotor motors 512 a, 514 a, 516 a, 518 a is less thanpower demand threshold 554 a.

Power management monitoring module 538 may also include an emergencydetection module 558, which detects an emergency event experienced byrotorcraft 500. For example, emergency detection module 558 may detectan engine loss event in which one or more engines 526 have reduced or nofunctionality. Power management command module 540 may allocate powerfrom battery 528 to electrical aircraft equipment 530 in response toemergency detection module 558 detecting the engine loss event. Thus,electrical aircraft equipment 530, which may normally receive power fromgenerator 524, may continue to function during the engine loss event byreceiving power from battery 528. Power management command module 540may also allocate full power to one or more tail rotor motors 512 a, 514a, 516 a, 518 a in response to emergency detection module 558 detectingthe engine loss event. It will be appreciated by one of ordinary skillin the art that any combination of modules 542, 546, 548, 550, 554, 558may be present or implemented by power management monitoring module 538.For example, power management monitoring module 538 may employ onlypower demand monitoring module 554. In embodiments in which tail rotors512, 514, 516, 518 are variable speed, battery 528 may also be rechargedby using regenerative energy from tail rotor motors 512 a, 514 a, 516 a,518 a during deceleration.

Power management system 532 reduces the weight and cost of electricallydistributed yaw control system 502 by allowing for the selection ofsmaller power components that still meet the power consumptionrequirements of electrically distributed yaw control system 502. Withcareful analysis of expected mission profiles, it may be possible toselect optimal component sizes to reduce the weight and cost of suchcomponents with little or no impact on the capability of rotorcraft 500to complete its mission. For example, using the illustrativeembodiments, generator 524 may be downsized from a larger generator suchas a 100 kilowatt generator to a lighter generator such as a 50 kilowattgenerator to reduce the overall weight and cost of rotorcraft 500 whilestill providing for the power demand requirements of yaw control system502. Bigger or additional batteries or generators may be offered as akit for helicopters with high power requirements. Power managementsystem 532 also reduces the demand on engine 526 to provide a longerengine lifespan and allows rotorcraft 500 to stay in high power demandconditions for a longer period of time.

Referring to FIG. 21 in the drawings, a power management system isschematically illustrated and generally designated 562. Power managementsystem 562 includes power distribution unit 564, which allocates powerbetween one or more tail rotor motors 566, generator 568 and battery570. Battery 570 provides power to electrical aircraft equipment 572.The power system of power management system 562 includes one or more yawcontrol system batteries 574 in addition to battery 570. In contrast tobattery 570, yaw control system battery 574 provides power only to tailrotor motors 566 and does not provide power to other systems of therotorcraft such as electrical aircraft equipment 572. Yaw control systembattery 574 allows power management system 562 to have additional powerat its disposal should it be needed to power tail rotor motors 566. Inother embodiments, power management system 562 may contain additionalhybrid batteries that serve both tail rotor motors 566 and other systemsof the rotorcraft.

Referring to FIGS. 22A-22B in the drawings, methods for managing powerfor an electrically distributed yaw control system of a rotorcraft aredepicted. In FIG. 22A, the method includes monitoring one or more flightparameters of a rotorcraft (step 578). The method also includesallocating power between a power system and one or more tail rotormotors based on the one or more flight parameters of the rotorcraft(step 580). In FIG. 22B, the method includes determining whether therotorcraft is in forward flight mode (step 582). If the methoddetermines that the rotorcraft is not in forward flight mode, the methodskips to step 590. If the method determines that the rotorcraft is inforward flight mode, the method allocates power from the generator tothe one or more tail rotor motors (step 584), allocates substantiallyzero power from the one or more batteries to the one or more tail rotormotors (step 586) and allocates power from the generator to the one ormore batteries to recharge the one or more batteries (step 588). Themethod then determines whether the rotorcraft is in hover mode (step590). If the method determines that the rotorcraft is not in hover mode,the method returns to step 582. If the method determines that therotorcraft is in hover mode, the method allocates power from thegenerator and the one or more batteries to the one or more tail rotormotors (step 592).

Referring to FIG. 23 in the drawings, rotorcraft 600 is depicted as ahelicopter including yaw control system 602. Yaw control matrix 604 iscoupled to an aft portion of tailboom 606 of rotorcraft 600. Yaw controlmatrix 604 includes shroud 608, which forms ducts 610. Upper forwardtail rotor 612, upper aft tail rotor 614, lower forward tail rotor 616and lower aft tail rotor 618 are disposed in ducts 610 and secured bystators 620. In the illustrated embodiment, shroud 608, ducts 610 andtail rotors 612, 614, 616, 618 collectively form a generally rhombusshape configuration, although shroud 608, ducts 610 and tail rotors 612,614, 616, 618 may separately or collectively form any shapeconfiguration. It will be appreciated by one of ordinary skill in theart that tail rotors 612, 614, 616, 618 may be positioned in an infinitenumber of configurations relative to one another depending on theembodiment, and that yaw control matrix 604 may include any number ofducts and tail rotors. In the illustrated embodiment, yaw control system602 is an electrically distributed yaw control system in which the tailrotor blades of tail rotors 612, 614, 616, 618 are fixed pitch tailrotor blades and motors 612 a, 614 a, 616 a, 618 a of tail rotors 612,614, 616, 618 are variable rotational speed motors capable of changingRPMs. In other embodiments, however, the tail rotor blades of tailrotors 612, 614, 616, 618 may be variable pitch tail rotor blades havinga fixed or variable rotational speed and may be electrically,mechanically or hydraulically driven. In yet other embodiments, one setof one or more tail rotors 612, 614, 616, 618 may be fixed pitch,variable rotational speed tail rotors while a different set of one ormore tail rotors 612, 614, 616, 618 are variable pitch, fixed rotationalspeed tail rotors. Electrically distributed yaw control system 602 mayhave dual or multiple channels that are able to crosstalk and shareinformation regarding the status and power consumption of tail rotormotors 612 a, 614 a, 616 a, 618 a in real time.

In traditional helicopters, thrust and power consumption do not alwayshave a linear relationship and may be difficult to test and predict.Thus, if multiple tail rotors are utilized, power imbalances may occurduring flight in which one tail rotor is overworked while another is notoperating at full capacity. In addition, the operating loads on the tailrotors of a dual or multiple tail rotor system may become imbalanced asthe flight regime or other flight parameters of the rotorcraft changeduring flight. Rotorcraft 600 includes a tail rotor balancing system 622to address these issues with real-time monitoring of certain flightparameters of rotorcraft 600 and operating parameters of tail rotors612, 614, 616, 618, which are used to balance power consumption, loadand other operating parameters of tail rotors 612, 614, 616, 618 duringflight. Tail rotor balancing system 622 includes a tail rotor balancingmodule 624 implemented by flight control computer 626. In otherembodiments, flight control computer 626 may include one or moreseparate motor control units and tail rotor balancing module 624 may beimplemented by the one or more separate motor control units.

Tail rotor balancing module 624 includes a tail rotor balancingmonitoring module 628, which monitors one or more parameters ofrotorcraft 600 such as one or more flight parameters of rotorcraft 600or one or more operating parameters of tail rotors 612, 614, 616, 618.Tail rotor balancing monitoring module 628 also identifies one or moretail rotors 612, 614, 616, 618 based on the one or more monitoredparameters. A tail rotor balancing command module 630 modifies one ormore operating parameters of the one or more tail rotors 612, 614, 616,618 identified by the tail rotor balancing monitoring module 628. Tailrotor balancing command module 630 may also modify one or more operatingparameters of one or more tail rotors 612, 614, 616, 618 that are notidentified by tail rotor balancing monitoring module 628. For example,during different maneuvers in flight, if one or more tail rotor motors612 a, 614 a, 616 a, 618 a are detected consuming more power than theother tail rotor motors due to, for example, airflow, air density, motorusage or other factors which result in the exceedance of a powergeneration or power consumption threshold, tail rotor balancing commandmodule 630 may activate a balance mode to limit power consumption forthe overconsuming tail rotor motor(s) and increase the rotational speedof one or more of the remaining tail rotors 612, 614, 616, 618 tomaintain the desired moment on the fuselage of rotorcraft 600. Sinceeach tail rotor 612, 614, 616, 618 is in a different position in yawcontrol matrix 604 and therefore exerts a different moment arm on thefuselage of rotorcraft 600, a given thrust for each tail rotor 612, 614,616, 618 translates to a different moment on the fuselage of rotorcraft600. Tail rotor balancing command module 630 takes into account thesespatial differences between tail rotors 612, 614, 616, 618 whenselecting one or more tail rotors to compensate for the reduced thrustresulting from slowing down or shutting off the overconsuming tail rotormotor(s). Tail rotor balancing command module 630 may include a yawmaintenance module 632 that selects the compensating tail rotor(s) anddetermines the extent to which the compensating one or more tail rotorsincrease their thrust based on the predetermined or desired moment onthe rotorcraft fuselage.

Tail rotor balancing monitoring module 628 includes a tail rotoroperating parameter monitoring module 634 to monitor and detect variousoperating parameters of tail rotors 612, 614, 616, 618 that may be usedas the basis for tail rotor balancing command module 630 to balance orotherwise modify certain operating parameters of tail rotors 612, 614,616, 618. Tail rotor operating parameter monitoring module 634 includesa rotational speed monitoring module 636 that monitors the rotationalspeeds of tail rotors 612, 614, 616, 618 and identifies one or more tailrotors 612, 614, 616, 618 having a rotational speed that exceeds arotational speed threshold 636 a. Tail rotor balancing command module630 may then reduce the rotational speed(s) of one or more tail rotors612, 614, 616, 618 having a rotational speed exceeding rotational speedthreshold 636 a, thereby preventing these tail rotors from overconsumingpower and being susceptible to wear or damage. To maintain the desiredmoment on the fuselage of rotorcraft 600, tail rotor balancing commandmodule 630 may compensate for the reduced thrust resulting from reducingthe rotational speed(s) of the tail rotor(s) exceeding rotational speedthreshold 636 a by increasing the rotational speed(s) of one or moredifferent tail rotors 612, 614, 616, 618. For example, tail rotors thathave not been slowed down by tail rotor balancing command module 630 maybe sped up proportional to their respective current rotational speeds.

Each tail rotor motor 612 a, 614 a, 616 a, 618 a consumes differentlevels of power based on their position in yaw control matrix 604 aswell as other factors, which allows power consumption to be used as acriterion for balancing tail rotors 612, 614, 616, 618. Thus, tail rotoroperating parameter monitoring module 634 includes a power consumptionmonitoring module 638 that monitors the power consumption of each tailrotor 612, 614, 616, 618. In some embodiments, power consumptionmonitoring module 638 may identify one or more tail rotors 612, 614,616, 618 having a power consumption that exceeds a maximum motor powerallowance 638 a. Tail rotor balancing command module 630 may then reducethe power consumption of the one or more tail rotors 612, 614, 616, 618that exceed maximum motor power allowance 638 a, which provides headroomfor allowing an increase in the power consumption of another tail rotor.To maintain yaw control thrust, tail rotor balancing command module 630may also increase the power consumption, and thus the rotational speed,of one or more tail rotors 612, 614, 616, 618 that were not identifiedby power consumption monitoring module 638 as exceeding maximum motorpower allowance 638 a.

In other embodiments, power consumption monitoring module 638 maydetermine whether tail rotors 612, 614, 616, 618 have a collective powerconsumption that exceeds a total generated power threshold 638 b. Tailrotor balancing monitoring module 628 may also identify one or more tailrotors 612, 614, 616, 618 in response to the collective powerconsumption of tail rotors 612, 614, 616, 618 exceeding total generatedpower threshold 638 b, and tail rotor balancing command module 630 mayreduce the power consumption of the tail rotor(s) identified by tailrotor balancing monitoring module 628. The identification of which tailrotors to be stopped or slowed down when total generated power threshold638 b is exceeded may be performed based on several factors. Forexample, tail rotor balancing command module 630 may reduce the powerconsumption of the highest consuming or least efficient tail rotor(s)612, 614, 616, 618. Tail rotor balancing command module 630 may alsoslow down or stop the one or more tail rotors 612, 614, 616, 618 thathave the least effective anti-torque or pro-torque thrust based on theoperation being performed and the position of each tail rotor 612, 614,616, 618.

Tail rotor operating parameter monitoring module 634 includes a torquemonitoring module 640, which monitors the torque of each tail rotor 612,614, 616, 618. Torque monitoring module 640 identifies one or more tailrotors 612, 614, 616, 618 having a torque that exceeds a torquethreshold 640 a. Tail rotor balancing command module 630 may then reducethe torque of the one or more tail rotors 612, 614, 616, 618 that exceedtorque threshold 640 a, and may increase the torque or rotational speedof one or more remaining tail rotors 612, 614, 616, 618 that have notexceeded torque threshold 640 a. Tail rotor operating parametermonitoring module 634 includes a motor failure detection module 642 thatdetects and identifies one or more tail rotors 612, 614, 616, 618experiencing a motor failure. Motor failure detection module 642 maydetect any mechanical or other potential failure issues of tail rotormotors 612 a, 614 a, 616 a, 618 a so that power balancing may betriggered either at the time or before the motor fails. In the event ofa motor failure, the remaining one or more motors automaticallycompensate for the failed motor to provide or sustain the desired momenton the fuselage of rotorcraft 600. For example, tail rotor balancingcommand module 630 may deactivate one or more tail rotors 612, 614, 616,618 for which motor failure detection module 642 has detected a motorfailure. Tail rotor balancing command module 630 may also increase therotational speed of one or more non-failing tail rotors to maintain adesired yaw control thrust.

Tail rotor balancing monitoring module 628 includes a load monitoringmodule 644 to monitor loads experienced by each tail rotor 612, 614,616, 618. For example, load monitoring module 644 may detect the loadand/or vibrations on the tail rotor blades of tail rotors 612, 614, 616,618 using optical sensors, Hall Effect sensors, strain sensors,accelerometers or other sensor types. Load monitoring module 644identifies one or more tail rotors 612, 614, 616, 618 experiencing aload exceeding a tail rotor load threshold 644 a. Tail rotor balancingcommand module 630 reduces the load on the tail rotor(s) identified byload monitoring module 644 as exceeding tail rotor load threshold 644 a.Load reduction may be performed in numerous ways such as by reducingpower consumption, rotational speed or blade pitch. Tail rotor balancingcommand module 630 may also increase the load on one or more tail rotors612, 614, 616, 618 that have not been identified by load monitoringmodule 644 as exceeding tail rotor load threshold 644 a.

Tail rotor balancing monitoring module 628 includes a flight parametermonitoring module 646, which monitors various flight parameters ofrotorcraft 600. In yaw control system 602, the aerodynamic environmentof each tail rotor 612, 614, 616, 618 differs for various flightregimes. For example, some tail rotors may experience higher loads ordifferent excitation frequencies than others depending on the flightmode and the anti-torque thrust requirement of rotorcraft 600. Flightparameter monitoring module 646 works in conjunction with tail rotorbalancing command module 630 to share the total yaw control thrustrequirement between tail rotors 612, 614, 616, 618 of yaw control matrix604 in a manner that balances and/or reduces the operating loads fortail rotors 612, 614, 616, 618. Sharing of the total thrust demand maybe dependent on the flight regime of rotorcraft 600 and may be performedin a manner that improves the overall load on yaw control matrix 604,which may reduce the noise produced by yaw control matrix 604 andimprove the dynamics and loads experienced by the airframe of rotorcraft600.

Flight parameter monitoring module 646 includes a flight mode monitoringmodule 648, which detects the flight mode of rotorcraft 600 usingsensors or manual input. Rotorcraft 600 may be capable of various flightmodes including hover mode and forward flight mode. Non-limitingexamples of parameters that may be used to determine the flight mode ofrotorcraft 600 include airspeed, climb rate, anti-torque requirement,main rotor torque and/or center of mass torque. Tail rotor balancingmonitoring module 628 identifies one or more tail rotors 612, 614, 616,618 based on the flight mode identified by flight mode monitoring module648 so that tail rotor balancing command module 630 may reduce therotational speed(s) or modify other operating parameters of theidentified tail rotor(s). Tail rotor balancing command module 630 mayalso deactivate the one or more tail rotors 612, 614, 616, 618identified by tail rotor balancing monitoring module 628 based on theflight mode detected by flight mode monitoring module 648. Additionally,tail rotor balancing command module 630 may activate or increase therotational speed of one or more tail rotors 612, 614, 616, 618 that havenot been slowed down or deactivated. Any combination of one, two, threeor more tail rotors 612, 614, 616, 618 may be slowed down, deactivated,activated or sped up for a particular flight mode or other flightparameter monitored by flight parameter monitoring module 646. The logicthat is used by tail rotor balancing module 624 to determine which tailrotors 612, 614, 616, 618 to slow down, deactivate, activate or speed upfor a particular flight mode may be programmed based on flight testexperience or may be actively controlled with a feedback loop usinginformation from yaw control matrix 604 or the main rotor of rotorcraft600.

Referring to FIGS. 24A-24D in conjunction with FIG. 23 , rotorcraft 600is shown in hover mode in FIGS. 24A-24B and forward flight mode in FIGS.24C-24D. In the non-limiting example of FIGS. 24A-24B, flight modemonitoring module 648 has detected that rotorcraft 600 is in hover modeand tail rotor balancing monitoring module 628 has identified upper andlower aft tail rotors 614, 618 based on the detected hover mode so thattail rotor balancing command module 630 may deactivate upper and loweraft tail rotors 614, 618. Tail rotor balancing command module 630 hasalso activated upper and lower forward tail rotors 612, 616. Upper andlower forward tail rotors 612, 616 may be slowed down or sped up toprovide a desired yaw control thrust. One reason for using upper andlower forward tail rotors 612, 616 and deactivating upper and lower afttail rotors 614, 618 is that upper and lower aft tail rotors 614, 618may experience excessive excitation in lower speed conditions such ashover mode. The operational condition of yaw control matrix 604 shown inFIG. 24B may also be used when rotorcraft 600 is in forward flight modeat lower speeds. In another non-limiting example, lower forward tailrotor 616 and lower aft tail rotor 618 may be slowed down or deactivatedand upper forward tail rotor 612 and upper aft tail rotor 614 may beactivated or sped up in the hover mode of rotorcraft 600. It will beappreciated by one of ordinary skill in the art, however, that thedetermination of which tail rotors 612, 614, 616, 618 to slow down,deactivate, activate or speed up in each flight mode including hovermode or low speed forward flight may be determined by flight testing,real-time feedback from yaw control matrix 604 or other factors.

When flight mode monitoring module 648 detects that rotorcraft 600 hastransitioned to a forward flight mode having a relatively higher forwardairspeed 650 as shown in FIG. 24C, the operational parameters of tailrotors 612, 614, 616, 618 are changed by tail rotor balancing commandmodule 630 as shown in FIG. 24D. In particular, in response to flightmode monitoring module 648 detecting that rotorcraft 600 is in forwardflight mode, tail rotor balancing monitoring module 628 has identifiedupper forward tail rotor 612 and upper aft tail rotor 614 to bedeactivated by tail rotor balancing command module 630. Lower forwardtail rotor 616 and lower aft tail rotor 618 have remained activated inforward flight mode and tail rotor balancing command module 630 may slowdown or speed up tail rotors 616, 618 to provide a desired yaw controlthrust. In one non-limiting example, the identification of upper forwardtail rotor 612 and upper aft tail rotor 614 as candidates for beingdeactivated in forward flight mode may be based on prior flight testingthat shows upper tail rotors 612, 614 to be problematic in forwardflight mode. Thus, lower tail rotors 616, 618 are relied upon to providethe required yaw control thrust in forward flight mode. It will beappreciated by one of ordinary skill in the art, however, that anycombination of one, two, three or more tail rotors 612, 614, 616, 618 inany position on yaw control matrix 604 may be slowed down, deactivated,activated or sped up in any flight mode detected by flight modemonitoring module 648 including hover mode, low speed forward flightmode and high speed forward flight mode.

Flight mode monitoring module 648 includes an airspeed monitoring module652, which monitors and detects airspeed 650 of rotorcraft 600. Tailrotor balancing monitoring module 628 identifies one or more tail rotors612, 614, 616, 618 in response to airspeed 650 of rotorcraft 600 eitherexceeding or being less than an airspeed threshold. Tail rotor balancingcommand module 630 may then slow down, deactivate, activate or speed upthe tail rotor(s) identified by tail rotor balancing monitoring module628 based on airspeed 650 of rotorcraft 600. By way of non-limitingexample, if airspeed 650 is less than the airspeed threshold, such as inhover mode or low speed forward flight mode, tail rotor balancingcommand module 630 may activate lower forward tail rotor 616 and upperaft tail rotor 614 and either slow down or deactivate upper forward tailrotor 612 and lower aft tail rotor 618. If airspeed 650 exceeds theairspeed threshold, tail rotor balancing command module 630 may balancethe load on yaw control matrix 604 by activating or speeding up upperforward tail rotor 612 and lower aft tail rotor 618 and slowing down ordeactivating lower forward tail rotor 616 and upper aft tail rotor 614.It will be appreciated by one of ordinary skill in the art, however,that any combination of one, two, three or more tail rotors 612, 614,616, 618 may be slowed down, deactivated, activated or sped up based onairspeed 650 either exceeding or being less than the airspeed threshold.

Returning to FIG. 23 , flight parameter monitoring module 646 includes amaneuver detection module 654, which detects the maneuver beingperformed by rotorcraft 600. Maneuver detection module 654 may detectthe maneuver being performed by rotorcraft 600 using sensor data fromon-board sensors. In other embodiments, the maneuver being performed byrotorcraft 600 may be determined based on the power consumption of eachtail rotor 612, 614, 616, 618. The maneuver may also be manuallyinputted by a pilot or from elsewhere. Non-limiting examples ofmaneuvers that may be detected by maneuver detection module 654 includesideward flight maneuvers, quick or sharp turns, climbs or descents.Tail rotor balancing monitoring module 628 may identify one or more tailrotors 612, 614, 616, 618 based on the maneuver detected by maneuverdetection module 654, and the identified tail rotor(s) may be sloweddown, deactivated, activated or sped up by tail rotor balancing commandmodule 630 accordingly. For example, tail rotor balancing command module630 may shut down certain tail rotors during particular maneuvers whileincreasing the rotational speeds of other tail rotors of yaw controlmatrix 604. High load imbalances may be more likely to occur betweentail rotors 612, 614, 616, 618 during certain maneuvers such as rightsideward flight. Tail rotor balancing command module 630 may thus adjustthe rotational speed and activation statuses of each tail rotor 612,614, 616, 618 during sideward flight maneuvers to rectify such loadimbalances. Should all tail rotors 612, 614, 616, 618 be needed for thesideward flight maneuver, tail rotor balancing command module 630 mayfactor in the load experienced by each tail rotor 612, 614, 616, 618,which may be detected by load monitoring module 644, to determine therespective speed of each tail rotor 612, 614, 616, 618. Since eachmaneuver of rotorcraft 600 requires different levels of yaw controlthrust from yaw control matrix 604, the amount of yaw control thrustrequired by the particular maneuver is a factor in determining whichtail rotors to slow down, deactivate, activate or speed up to ensurethat rotorcraft 600 can continue to successfully perform the maneuver.If it is determined that the maneuver cannot continue to be performedbased on the power consumption or load experienced by tail rotors 612,614, 616, 618, an alert may be sent to the operator of rotorcraft 600 bypilot alert module 656 to notify the operator to discontinue performingthe current maneuver. Pilot alert module 656 may be particularly usefulfor maneuvers that require high levels of anti-torque thrust such assideward flight.

Another flight parameter that may be used to determine when to triggerdifferent tail rotors 612, 614, 616, 618 to be slowed down, deactivated,activated or sped up is airframe vibration or instability. Thus, flightparameter monitoring module 646 includes an airframe stabilitymonitoring module 658 that monitors the excitation frequency(ies) oftail rotors 612, 614, 616, 618. Tail rotor balancing monitoring module628 identifies one or more tail rotors 612, 614, 616, 618 in response tothe excitation frequency of any combination of tail rotors 612, 614,616, 618 approximating a natural frequency of any airframe component ofrotorcraft 600. Tail rotor balancing command module 630 may then slowdown, deactivate, activate or speed up the tail rotor(s) identified bytail rotor balancing monitoring module 628 so that the excitationfrequency(ies) of tail rotors 612, 614, 616, 618 no longerapproximate(s) the natural frequency of the airframe component. Forexample, if tail rotors 612, 614, 616, 618 are rotating at a naturalfrequency of tailboom 606 or other airframe structure, tail rotorbalancing command module 630 may slow down or deactivate one or moretail rotors 612, 614, 616, 618 and increase the rotational speeds ofothers so that they no longer excite the natural frequency of theairframe, thereby preserving airframe integrity and reducing vibrationloads. Flight parameter monitoring module 646 may also monitor otherflight parameters of rotorcraft 600 such as ambient air density,altitude, environmental flight conditions or others, and such flightparameters may be used as the basis for slowing down, deactivating,activating or speeding up any combination of tail rotors 612, 614, 616,618 in yaw control matrix 604 to balance loads. Minimizing the loads ontail rotors 612, 614, 616, 618 allows for a lighter yaw control matrix604 and increases the fatigue life of the components of yaw controlmatrix 604 to reduce aircraft operating cost.

Referring to FIGS. 25A-25B in the drawings, isometric views of anelectric vertical takeoff and landing (eVTOL) aircraft 662 for use withbalancing system 622 are depicted. FIG. 25A depicts eVTOL aircraft 662in a forward flight mode wherein the rotor systems provide forwardthrust with the forward airspeed of eVTOL aircraft 662 providingwing-borne lift enabling eVTOL aircraft 662 to have a high speed and/orhigh endurance forward flight mode. FIG. 25B depicts eVTOL aircraft 662in a VTOL flight mode wherein the rotor systems provide thrust-bornelift. VTOL flight mode includes takeoff, hover and landing phases offlight. In the illustrated embodiment, eVTOL aircraft 662 includes wings664 a, 664 b. Wings 664 a, 664 b have an airfoil cross-section thatgenerates lift responsive to the forward airspeed of eVTOL aircraft 662.

In the illustrated embodiment, eVTOL aircraft 662 includes four rotorsystems forming a two-dimensional distributed thrust array. The thrustarray of eVTOL aircraft 662 includes a forward-port rotor system 666 a,a forward-starboard rotor system 666 b, an aft-port rotor system 666 cand an aft-starboard rotor system 666 d, which may be referred tocollectively as rotor systems 666. Forward-port rotor system 666 a andforward-starboard rotor system 666 b are each rotatably mounted to ashoulder portion of the fuselage at a forward station thereof. Aft-portrotor system 666 c is rotatably mounted on the outboard end of wing 664a. Aft-starboard rotor system 666 d is rotatably mounted on the outboardend of wing 664 b. In the illustrated embodiment, rotor systems 666 areducted rotor systems each having a five bladed rotor assembly withvariable pitch rotor blades operable for collective pitch control. Rotorsystems 666 may each include at least one variable speed electric motorand a speed controller configured to provide variable speed control tothe rotor assembly over a wide range of rotor speeds, or alternativelymay each include at least one constant speed electric motor to providefixed RPM. In other embodiments, the rotor systems could be non-ductedor open rotor systems, the number of rotor blades could be eithergreater than or less than five and/or the rotor blades could have afixed pitch. eVTOL aircraft 662 may include any number of rotor systemseither greater than or less than four rotor systems such as a coaxialrotor system or six rotor systems.

When eVTOL aircraft 662 is operating in the forward flight orientationand supported by wing-borne lift, rotor systems 666 each have agenerally vertical position with the forward rotor assemblies rotatinggenerally in a forward vertical plane and the aft rotor assembliesrotating generally in an aft vertical plane, as best seen in FIG. 25A.When eVTOL aircraft 662 is operating in the VTOL orientation andsupported by thrust-borne lift, rotor systems 666 each have a generallyhorizontal position such that the rotor assemblies are rotating ingenerally the same horizontal plane, as best seen in FIG. 25B.Transitions between the VTOL orientation and the forward flightorientation of eVTOL aircraft 662 are achieved by changing the angularpositions of rotor systems 666 between their generally horizontalpositions and their generally vertical positions.

Balancing module 624 of FIG. 23 may, in other embodiments, beimplemented to balance the power consumption, rotational speeds and/orloads of rotor systems 666. For example, should one or more rotorsystems 666 experience a power consumption, rotational speed, torque orload that exceeds a predetermined threshold, a different set of one ormore rotor systems 666 may compensate for the overworked rotor system toprovide an overall desired propulsion thrust in either flight mode. Inaddition, each rotor system 666 may be slowed down, deactivated,activated or sped up based on the flight mode, airspeed, maneuver,excitation frequency or natural frequency of eVTOL aircraft 662 tobalance or improve the overall loads experienced by rotor systems 666.Indeed, balancing module 624 is not limited only to yaw control systems,as its applicability to eVTOL aircraft 662 demonstrates its wide rangeof applications on any rotorcraft having two or more rotors.

Referring to FIGS. 26A-26C in the drawings, various methods foroperating a plurality of tail rotors of a rotorcraft such as ahelicopter are depicted. In FIG. 26A, the method includes monitoring oneor more parameters of a rotorcraft (step 670). The method includesidentifying a first set of one or more tail rotors based on the one ormore parameters of the rotorcraft (step 672). The method includesmodifying one or more operating parameters of the first set of one ormore tail rotors (step 674). In some embodiments, step 674 may includereducing a rotational speed of the first set of one or more tail rotorsand increasing a rotational speed of a second set of different one ormore tail rotors to maintain a moment on the fuselage of the rotorcraft.In other embodiments, the method may include modifying one or moreoperating parameters of a second set of one or more tail rotors that isdifferent from the first set of one or more tail rotors. In FIG. 26B,the method includes monitoring an airspeed of a rotorcraft (step 676).The method includes identifying a first set of one or more tail rotorsbased on the airspeed of the rotorcraft (step 678). The method includesdeactivating the first set of one or more tail rotors (step 680). InFIG. 26C, the method includes monitoring power consumption of each ofthe tail rotors (step 682). The method includes identifying a first setof one or more tail rotors having a power consumption exceeding amaximum motor power allowance (step 684). The method includes reducingthe power consumption of the first set of one or more tail rotors (step686). The method also includes increasing the power consumption of oneor more remaining tail rotors not in the first set of one or more tailrotors (step 688).

The flowcharts and block diagrams in the different depicted embodimentsillustrate the architecture, functionality, and operation of somepossible implementations of apparatus, methods and computer programproducts. In this regard, each block in the flowchart or block diagramsmay represent a module, segment, or portion of code, which comprises oneor more executable instructions for implementing the specified functionor functions. In some alternative implementations, the function orfunctions noted in the block may occur out of the order noted in thefigures. For example, in some cases, two blocks shown in succession maybe executed substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved.

The flight control computers of the illustrative embodiments includecomputing elements, or modules, such as non-transitory computer readablestorage media that include computer instructions executable byprocessors for controlling flight operations. The computing elements maybe implemented as one or more general-purpose computers, special purposecomputers or other machines with memory and processing capability. Thecomputing elements may include one or more memory storage modulesincluding, but is not limited to, internal storage memory such as randomaccess memory, non-volatile memory such as read only memory, removablememory such as magnetic storage memory, optical storage, solid-statestorage memory or other suitable memory storage entity. The computingelements may be implemented as microprocessor-based systems operable toexecute program code in the form of machine-executable instructions. Thecomputing elements may be selectively connectable to other computersystems via a proprietary encrypted network, a public encrypted network,the Internet or other suitable communication network that may includeboth wired and wireless connections.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. A yaw control system for a helicopter having anairframe including a tailboom comprising: one or more tail rotorsrotatably coupled to the tailboom, the one or more tail rotors havingone or more operating parameters, each tail rotor including a pluralityof tail rotor blades; one or more load sensors coupled to at least oneof the airframe or the tail rotor blades of the helicopter; and a flightcontrol computer implementing an airframe protection module, theairframe protection module comprising: an airframe protection monitoringmodule configured to monitor one or more flight parameters of thehelicopter, the airframe protection monitoring module including a loaddetermination module configured to detect a load on the at least one ofthe airframe or the tail rotor blades of the helicopter using the one ormore load sensors; and an airframe protection command module configuredto modify the one or more operating parameters of the one or more tailrotors based on the one or more flight parameters of the helicopterincluding the load detected by the load determination module, theairframe protection command module limiting anti-torque input from anoperator of the helicopter when the load detected by the loaddetermination module exceeds a load threshold, thereby protecting theairframe of the helicopter.
 2. The yaw control system as recited inclaim 1 wherein the airframe protection monitoring module furthercomprises an airspeed monitoring module configured to monitor anairspeed of the helicopter, the airframe protection command moduleconfigured to modify the one or more operating parameters of the one ormore tail rotors based on the airspeed of the helicopter.
 3. The yawcontrol system as recited in claim 1 wherein the airframe protectionmonitoring module further comprises a maneuver detection moduleconfigured to detect a maneuver being performed by the helicopter, theairframe protection command module configured to modify the one or moreoperating parameters of the one or more tail rotors based on themaneuver.
 4. The yaw control system as recited in claim 1 wherein theone or more load sensors are coupled to the airframe of the helicopterand the load determination module detects the load on the airframe ofthe helicopter using the one or more load sensors, the airframeprotection command module configured to modify the one or more operatingparameters of the one or more tail rotors based on the load on theairframe.
 5. The yaw control system as recited in claim 1 wherein theone or more load sensors are coupled to the tail rotor blades of thehelicopter and the load determination module detects the load on thetail rotor blades using the one or more load sensors, the airframeprotection command module configured to modify the one or more operatingparameters of the one or more tail rotors based on the load on the tailrotor blades.
 6. The yaw control system as recited in claim 1 whereinthe airframe protection monitoring module further comprises a tail rotorblade clearance monitoring module configured to detect a clearancedistance between the tail rotor blades and the airframe of thehelicopter, the airframe protection command module configured to modifythe one or more operating parameters of the one or more tail rotorsbased on the clearance distance.
 7. The yaw control system as recited inclaim 6 wherein the airframe protection command module is configured tomodify the one or more operating parameters of the one or more tailrotors in response to the clearance distance being less than a minimumtail rotor blade clearance threshold.
 8. The yaw control system asrecited in claim 1 wherein the airframe protection command modulefurther comprises an acceleration regulator to modify a maximum angularacceleration of the one or more tail rotors, the acceleration regulatorlimiting angular acceleration of the one or more tail rotors to themaximum angular acceleration based on the one or more flight parametersof the helicopter.
 9. The yaw control system as recited in claim 1wherein the airframe protection command module further comprises anacceleration regulator configured to modify a maximum angulardeceleration of the one or more tail rotors, the acceleration regulatorlimiting angular deceleration of the one or more tail rotors to themaximum angular deceleration based on the one or more flight parametersof the helicopter.
 10. The yaw control system as recited in claim 1wherein the airframe protection command module further comprises arotational speed regulator configured to modify a maximum rotationalspeed of the one or more tail rotors, the rotational speed regulatorlimiting a rotational speed of the one or more tail rotors to themaximum rotational speed based on the one or more flight parameters ofthe helicopter.
 11. The yaw control system as recited in claim 1 whereinthe airframe protection command module further comprises a rotationalspeed regulator configured to reduce a rotational speed of the one ormore tail rotors based on the one or more flight parameters of thehelicopter.
 12. The yaw control system as recited in claim 1 wherein theone or more load sensors comprise a strain gauge.
 13. The yaw controlsystem as recited in claim 1 wherein the one or more load sensors arecoupled to the tailboom of the helicopter.
 14. The yaw control system asrecited in claim 1 wherein the one or more load sensors are coupled tothe tail rotor blades and the load determination module detects abending load on the tail rotor blades using the one or more loadsensors.
 15. A rotorcraft comprising: a fuselage; a tailboom extendingfrom the fuselage, the tailboom having an aft portion; and a yaw controlsystem comprising: a shroud coupled to the aft portion of the tailboomand forming one or more ducts; one or more tail rotors disposed in theone or more ducts, the one or more tail rotors having one or moreoperating parameters, each tail rotor including a plurality of tailrotor blades; one or more load sensors coupled to at least one of anairframe or the tail rotor blades of the rotorcraft; and a flightcontrol computer implementing an airframe protection module, theairframe protection module comprising: an airframe protection monitoringmodule configured to monitor one or more flight parameters of therotorcraft, the airframe protection monitoring module including a loaddetermination module configured to detect a load on the at least one ofthe airframe or the tail rotor blades of the rotorcraft using the one ormore load sensors; and an airframe protection command module configuredto modify the one or more operating parameters of the one or more tailrotors based on the one or more flight parameters of the rotorcraftincluding the load detected by the load determination module, theairframe protection command module limiting anti-torque input from anoperator of the rotorcraft when the load detected by the loaddetermination module exceeds a load threshold, thereby protecting theairframe of the rotorcraft.
 16. The rotorcraft as recited in claim 15wherein each tail rotor further comprises a motor secured by one or morestators within a respective one of the ducts, the airframe protectioncommand module configured to modify the one or more operating parametersof the one or more tail rotors based on the one or more flightparameters of the rotorcraft to prevent contact between the tail rotorblades and the one or more stators.
 17. The rotorcraft as recited inclaim 15 wherein the plurality of tail rotor blades comprise a pluralityof variable pitch tail rotor blades; and wherein, the airframeprotection command module further comprises a blade pitch regulatorconfigured to modify a blade pitch of the variable pitch tail rotorblades based on the one or more flight parameters of the rotorcraft. 18.A method for protecting an airframe of a helicopter comprising:monitoring one or more flight parameters of the helicopter includingdetecting a load on at least one of the airframe or tail rotor blades ofthe helicopter using one or more load sensors; and modifying one or moreoperating parameters of one or more tail rotors of the helicopter basedon the one or more flight parameters including the load on the at leastone of the airframe or the tail rotor blades of the helicopter, therebyprotecting the airframe of the helicopter; wherein, modifying the one ormore operating parameters of the one or more tail rotors of thehelicopter includes limiting anti-torque input from an operator of thehelicopter in response to the load exceeding a load threshold.
 19. Themethod as recited in claim 18 wherein monitoring the one or more flightparameters of the helicopter further comprises monitoring at least oneof an airspeed of the helicopter, a maneuver being performed by thehelicopter or a clearance distance between the airframe of thehelicopter and the tail rotor blades of the one or more tail rotors; andwherein, modifying the one or more operating parameters of the one ormore tail rotors of the helicopter further comprises modifying at leastone of a maximum angular acceleration, a maximum angular deceleration, amaximum rotational speed, a rotational speed or a blade pitch of the oneor more tail rotors.